Title of Invention	CATALYTIC DEPOLYMERIZATION OF POLYMERS CONTAINING ELECTROPHILIC LINKAGES USING NUCLEOPHILIC REAGENTS
Abstract	The present invetion to a method for carrying out depolymerization of a polymer containing electrophilic linkages in the presence of a catalyst and a nucleophilic reagent, wherein production of undesirable byproducts resulting from polymer degradation is minimized. The reaction can be carried out at a temperature of 80°C or less, and generally involves the use of an organic, nonmetallic catalyst, thereby ensuring that the deploymerization product(s) aer substantially free of metal contaminants. In an exemplary depolymerization method, the catalyst is a carbene compound such as an N-heterocyclic carbene, or is a precursor to a carbene compound. The method provides an important alternative to current recycling techniques such as those used in the degradation of polyesters, polyamides, and the like.

Title of Invention

CATALYTIC DEPOLYMERIZATION OF POLYMERS CONTAINING ELECTROPHILIC LINKAGES USING NUCLEOPHILIC REAGENTS

Abstract

The present invetion to a method for carrying out depolymerization of a polymer containing electrophilic linkages in the presence of a catalyst and a nucleophilic reagent, wherein production of undesirable byproducts resulting from polymer degradation is minimized. The reaction can be carried out at a temperature of 80°C or less, and generally involves the use of an organic, nonmetallic catalyst, thereby ensuring that the deploymerization product(s) aer substantially free of metal contaminants. In an exemplary depolymerization method, the catalyst is a carbene compound such as an N-heterocyclic carbene, or is a precursor to a carbene compound. The method provides an important alternative to current recycling techniques such as those used in the degradation of polyesters, polyamides, and the like.

Full Text	CATALYTIC DEFOLVMERIZATION OF FOLVMERS CONTAINING ELECTROPHILIC LINKAGES USING NUCLEOPHILIC REAGENTS ACKNOWLEDGEMENT of GOVERNMENT SUPPORT [0001] This invention was made iiJpart with Government support under a grant from the National Science Foundation (CoOTcrative Agreement No. DMR-980677). Accordingly, the Government may have certain pTghts to this invention. TECHNICAL FIELD [0002] This invention relates generally to the depolymerization of polymers, and, more particularly relates to an organocatalytic method for depolymerizing polymers using nucleophilic reagents.. The invention is applicable in numerous fields, including industrial chemistry and chemical waste disposal, plastics recycling, and manufacturing processes requiring a simple and convenient method for the degradation of polymers. BACKGROUND ART [0003] Technological advances of all kinds continue to present many complex ecological issues. Consequently, waste management and pollution prevention are two very significant challenges of the 21" century. The overwhelming quantity of plastic refuse has significantly contributed to the critical shortage of landfill space faced by many communities. For example, poly(ethylene terephthalate) (poly(oxy-l,2-ethanediyl-oxycarbonyl-l,4-diphenylenecarbonyl); "PET"), a widely used engineering thermoplastic for carpeting, clothing, tire cords, soda bottles and other containers, fihn, automotive applications, electronics, displays, etc. will contribute more than 1 billion pounds of waste to land-fills in 2002 alone. The worldwide production of PET has been growing at an annual rate of 10 % per year, and with the increase in use in electronic and automotive applications, this rate is expected to increase significantly to 15% per year. Interestingly, the precursor monomers represent only about 2% of the petrochemical stream. Moreover, the proliferation of the use of organic solvents, halogenated solvents, water, and energy consumption in addressing the need to recycle commodity polymers such as PET and other polyesters has created the need for envu-onmentally responsible and energy efficient recyclmg processes. See Nadkami (1999) International Fiber Journal 14(3). [0004] Significant effort has been invested in researching recycling strategies for PET, and these efforts have produced three commercial options; mechanical, chemical and energy recycling. Energy recycling simply bums the plastic for its calorific content. Mechanical recycling, the most widespread approach, involves grinding the polymer to powder, which is then mixed with "virgin" PET. See Mancini et al. (1999) Materials Research 2(l):33-38. Many chemical companies use this process in order to recycle PET at the rate of approximately 50,000 tons/year per plant. In Europe, all new packaging materials as of 2002 must contain 15% recycled material. However, it has been demonstrated that successive recycling steps cause significant polymer degradation, in turn resulting in a loss of desirable mechanical properties. Recycling using chemical degradation involves a process that depolymerizes a polymer to starting material, or at least to relative short oligomeric components. Clearly, this process is most desirable, but is the most difficult to control since elevated temperature and pressure are required along with a catalyst composed of a strong base, or an organometallic complex such as an organic titanate. See Sako et al. (1997) Proc. ofthe4' Int'l Symposium on Supercritical Fluids, ^^. 107-110. The use of such a catalyst results in significant quantities of undesirable byproducts, and materials processed by these methods are thus generally unsuitable for use in medical materials or food packaging, limiting their utility. Moreover, the energy required to effect depolymerization essentially eliminates sustainability arguments. [OOOS] Accordingly, there is a need in the art for an improved depolymerization . method. Ideally, such a method would not involve extreme reaction conditions, use of metallic catalysts, or a process that results in significant quantities of potentially problematic by-products. DISCLOSURE OF THE INVENTION [0006] The invention is directed to the aforementioned need in the art, and, as such, provides an efficient catalytic depolymerization reaction that employs mild conditions, wherein production of undesirable byproducts resultmg from polymer degradation is minimized. The reaction can be carried out at temperatures of at most 80 °C, and, because a nonmetaJlic catalyst is preferably employed, the depolymerization products, in a preferred embodiment, are substantially free of metal contaminants. With many of the caibene catalysts aisciosea nerein, ine uepuiynicnzaLiuii icia»,Lion can be carried out at a temperature of at most 60 °C or even 30 °C or lower, i.e., at room temperature. 10007] More specifically, in one aspect of the invention, a method is provided for depolymerizing a polymer having a backbone containing electrophilic linkages, wherein the method involves contacting the polymer with a nucleophilic reagent and a catalyst at a temperature of less than 80 °C. An important application of this method is in the depolymerization of polyesters, including homopolymeric polyesters (in which ali of the electrophilic linkages are ester linkages) and polyester copolymers (in which a fraction of the electrophilic linkages are ester linkages and the remainder of the electrophilic linkages are other than ester linkages). [0008] In a related aspect of the invention, a method is provided for depolymerizing a polymer having a backbone containing electrophilic linkages, wherein the method involves contacting the polymer with a nucleophilic reagent and a catalyst that yields depolymerization products that are substantially free of metal contaminants. The polymer may be, for example, a polyester, a polycarbonate, a polyurethane, or a related polymer, in either homopolymeric or copolymeric form, as indicated above. In this embodiment, in order to provide reaction products that are substantially free of contamination by metals and metal-containing compounds, the catalyst used is a purely organic, normietallic catalyst. Preferred catalysis herein are carbene compounds, which act as nucleophilic catalysts, as well as precursors to carbene compounds, as will be discussed injra. As is well understood in the art, carbenes are electronically neutral compounds containing a divalent carbon atom with only six electrons in its valence shell. Carbenes include, by way of example, cyclic diaminocarbenes, imida2ol-2-ylidenes (e.g., l,3-dimesityl-imidazol-2-ylidene and 1,3-dimesityl-4,5-dihydroiniidazoI-2-yIidene), J,2,4-lria2oI-3-y]idenes, and l,3-thiazo!-2-ylidenes; see Bourissou et al. (2000) Chem. Rev. 100:39-91. [0009] Since the initial description of the synthesis, isolation, and characterization of stable carbenes by Arduengo (Arduengo et al. (1991) J Am. Chem, Soc. 113:361; Arduengo et al. (1992) J. Am. Chem, Soc. J_U:5530), the exploration of their chemical reactivity has become a major area of research. See, e.g., Arduengo et al. (I999)^cc. Chem. Res. 32:913; Bourissou etal. (2000), jupra; and Brode (1995) yiwgew. Chem. Int. Ed. Eng. 34:1021. Although carbenes have now been extensively investigated, and have in fact been established as usefiil in many synthetically important reactions, there has been no disclosure or suggestion to use carbenes as catalysts in nucleophilic depolymerization reactions, i.e., reactions in which a polymer containing electrophilic lijikages is depolymerized with a nucleophilic reagent m the presence of a carbene catalyst. [00010] Suitable catalysts for use herein thus include heteroatom-stabilized carbenes or precursors to such carbenes. The heteroatom-stabilized carbenes have the structure of formula (I) wherein: [00011 ] E' and E^ are independently selected from N, NR^, O, P, PR^, and S, R^ is hydrogen, heteroalkyi, or heteroary], x and y are independently zero, I, or 2, and are selected to correspond to the valence state of E and E , respectively, and wherein when E and E^ are other than O or S, then E' and E may be linked through a linking moiety that provides a heterocyclic ring in which E' and E are incorporated as heteroatoms; [00012] R' and R^ are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30 hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl; [00013] L' and L^ are linkers containing 1 to 6 spacer atoms, and are independently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene; and [00014] m and n are independently zero or 1, such that L' and \} are optional. 100015] Certain carbene catalysts of formula (I) are novel chemical compounds and are claimed as such herein. These novel carbenes are those wherein a heteroatom is directly bound to E' and/or E^, and mclude, solely by way of example, carbenes of formula (I) wherein E' is NR^ and R^ is a heteroalkyi or heteroaryl group such as an alkoxy, alkylthio, aryloxy, arylthio, aralkoxy, or aralkylthio moiety. {00016] Carbene precursors suitable as catalysts herein include tri-substituted methanes having the structure of formula (PI), metal adducts having the structure of formula (PII), and tetrasubstituted olefins having the structure (PHI) wherein, in formulae (PI) and (PII): [00017] E^ and E^ are independently selected from N, NR^, O, P, PR^, and S, R^ is hydrogen, heteroalkyl, or heteroaryl, x and y are independently zero, 1, or 2, and are selected to correspond to the valence state of E' and E^, respectively^ and wherein when E' and E^ are other than O or S, then E' and E^ may be linked through a linking moiety that provides a heterocyclic ring in which E and E are incorporated as heteroatoms; [00018] R' and R^ are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C^-Cso hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl; [00019] L' and L^ are linkers containing 1 to 6 spacer atoms, and are independently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom- containing hydrocarbylene; [00020] m aiid n are independently zero or 1; [00021] R^ is selected from alkyl, heteroalkyl, aryl, heteroaryl, aralkyl, or heteroaralkyi, substituted with at least one electron-withdrawing substituent; [00022] M is a metal; [00023] Ln i5 a ligand; and [00024] j is the number of Hgands bound to M. [00025] In compounds of formula (PHI), the substituents are as follows: [00026] E^ and E" are defined as for E' and E^ [00027] V and w are defined as for x and y; [00028] R^ and R^ are defined as for R' and R^; [00029] L^ and L" are defined as for L' and L^ and [00030] h and k are defined as for m and n. [00031] The carbene precursors may be in the form of a salt, in which case the precursor is positively charged and associated with an anionic counterion, such as a halide ion (I, Br, CI), a hexafluorophosphate anion, or the like. [00032] Novel carbene precursors herein include compounds of formula (PI), those compounds of formula (PII) in which a heteroatom is directly bound to E' and/or E^, and those compounds of formula (PHI) in which a heteroatom is directly bound to at least one of E', E^ E^, and E^ and may be in the form of a salt as noted above. [00033] Ideally, the carbene catalyst used in conjunction with the present depolymerization reaction is an N-heterocycHc carbene having the structure of formula (II) [00034] R\ R^, L', L^, m, and n are as defined above; and [00035] L is a hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, or substituted heteroatom-contaming hydrocarbylene linker, wherein two or more substituents on adjacent atoms within L may be Jinked to form an additional cyclic group. [00036] As alluded to above, one important application of the present invention is in the recycling of polyesters, including, by way of illustration and not limitation: PET; poly (butylene terephthalate) (PBT); poly(alkylene adipate)s and their copolymers; and poly(e-caprolactone). The methodology of the invention provides an efficient means to degrade such polymers into their component monomers and/or relatively short oligomeric fragments without need for extreme reaction conditions or metallic catalysts. BRIEF DESCRIPTION OF THE DRAWINGS [00037J Figure I illustrates the organocalalytic depolymerization of PET in the presence of excess methanol using N-heterocyclic carbene catalyst, as evaluated in Example 7. {00038] Figure 2 illustrates the organocatalytic depolymerization of PET in the presence of ethylene glycol using N-heterocyclic carbene catalyst, as evaluated in Example 6. DETAILED DESCRIPTION OF THE INVENTION {00039\| Unless otherwise mdicated, this invention is not limited to specific polymers, carbene catalysts, nucleophilic reagents, or depolymerization conditions. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. [00040] As used in the specification and the appended claims, the singular forms "a," "an," and "the" mclude plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a polymer" encompasses a combination or mixtiu-e of different polymers as well as a single polymer, reference to "a catalyst" encompasses both a single catalyst as well as two or more catalysts used in combination, and the like. [00041] In this specification and in the claims that follow, reference will be made to a number of terms, which shall be defined to have the following meanings: [00042] As used herein, the phrase "having the formula" or "having the structure" is not intended to be limiting and is used in the same way that the term "comprising" is commonly used. [00043] The term "alkyl" as used herein refers to a linear, branched, or cyclic saturated hydrocarbon group typically although not necessarily containing 1 to about 20 carbon atoms, preferably 1 to about 12 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, f-butyl, octyl, decyl, and the like, as well as cycloalkyi groups such as cyclopentyl, cyclohexyl and the like. Generally, although again not necessarily, alkyl groups herein contain 1 to about 12 carbon atoms. The term "lower alkyl" intends an alkyl group of 1 to 6 carbon atoms, and the specific term "cycloalkyi" intends a cyclic alkyl group, typically having 4 to 8, preferably 5 to 7, carbon atoms. The term "substituted alkyl" refers to alkyl substituted with one or more substituent groups, and the terms "heteroatom-containing alkyl" and "heteroalkyl" refer to alkyl in which at least one carbon atom is replaced with a heteroatom. If not otherwise indicated, the terms "alkyl" and "lower alkyl" include Imear, branched, cyclic, unsubstituted, substituted, and/or heteroatom-containing alkyl and lower alky], respectively. [00044] The term "alkylene" as used herein refers to a difunctional linear, branched, or cyclic alkyl group, where "alkyl" is as defined above. [00045] The term "alkenyl" as used herein refers to a linear, branched, or cyclic hydrocarbon group of 2 to about 20 carbon atoms containing at least one double bond, such as ethenyl, n-propenyl, isopropenyl, n-butenyl, isobutenyl, octenyl, decenyl, tetradecenyl, hexadecenyl, eicosenyl, tetracosenyl, and the like. Preferred alkenyl groups herein contain 2 to about 12 carbon atoms. The term "lower alkenyl" intends an alkenyl group of 2 to 6 carbon atoms, and the specific term "cycloalkenyl" intends a cyclic alkenyl group, preferably havmg 5 to 8 carbon atoms. The term "substituted alkenyl" refers to alkenyl substituted with one or more substituent groups, and the terms "heteroatom-containing alkenyl" and "heteroaikenyl" refer to alkenyl in which at least one carbon atom is replaced with a heteroatom. If not otherwise indicated, the terms "alkenyl" and "lower alkenyl" include linear, branched, cycUc, unsubstituted, substituted, and/or heteroatom-containing alkenyl and lower alkenyl, respectively. [00046] The term "alkenylene" as used herein refers to a difunctional linear, branched, or cyclic alkenyl group, where "alkenyl" is as defined above. [00047] The term "alkoxy" as used herein refers to a group -O-alkyl wherein "alkyl" is as defined above, and the term "alkylthio" as used herein refers to a group -S-alkyl wherein "alkyl is as defined above. [00048] The term "aryl" as used herein, and unless otherwise specified, refers to an aromatic substituent containing a single aromatic ring or multiple aromatic rings that are fused together, directly linked, or indirectly linked (such that the different aromatic rings are bound to a common group such as a methylene or ethylene moiety). Preferred aryl groups contain 5 to 20 carbon atoms and either one aromatic ring or 2 to 4 fused or linked aromatic rings, e.g., phenyl, naphthyl, biphenyl, and the like, with more preferred aryl groups containing 1 to 3 aromatic rings, and particularly preferred aryl groups containing 1 or 2 aromatic rings and 5 to 14 carbon atoms. "Substituted aryl" refers to an aryl moiety substituted with one or more substituent groups, and the terms "heteroatom-containing aryl" and "heteroaryl" refer to aryl in which at least one carbon atom is replaced with a heteroatom. Unless otherwise indicated, the terms "aromatic," "aryl," and "arylene" include heteroaromatic, substituted aromatic, and substituted heteroaromatic species. [00049] The term "aryloxy" refers to a group -0-aryl wherein "aryl" is as defined above. [00050] The term "alkaryl" refers to an aryl group with at least one and typically 1 to 6 alkyl, preferably 1 to 3, aUcyl substituents, and the term "aralkyl" refers to an alkyl group with an aryl substituent, wherein "aryl" and "alkyl" are as defmed above. Alkaryl groups include, for example, p-methylphenyl, 2,4-dimethylphenyl, 2,4,6-trimethylphenyl, and the like. The term "aralkyl" refers to an alkyl group substituted with an ary] moiety, wherein "alkyl" and "aryl" are as defined above. [00051] The term "alkaryloxy" refers to a group -0-R wherein R is alkaryl, the term "alkarylthio" refers to a group -S-R wherein R is alkaryl, the term aralkoxy refers to a group -0-R wherein R is aralkyl, the term "aralkylthio" refers to a group -S-R wherein R is aralkyl. [00052] The terms "halo," "halide," and "halogen" are used in the conventional sense to refer to a chloro, bronio, fluoro, or iodo substituent. The terms "haloalkyl," "haloalkenyl," and "haloalkynyl" (or "halogenated alkyl," "halogenated alkenyl," and "halogenated alkynyl") refer to an alkyl, alkenyl, or alkynyl group, respectively, in which at least one of the hydrogen atoms in the group has been replaced with a halogen atom. [00053] "Hydrocarbyl" refers to univalent hydrocarbyl radicals containing 1 to about 30 carbon atoms, preferably 1 to about 20 carbon atoms, more preferably 1 to about 12 carbon atoms, including linear, branched, cyclic, saturated, and unsaturated species, such as alkyl groups, alkenyl groups, aryl groups, alkaryl groups, and the like. The term "lower hydrocarbyl" intends a hydrocarbyl group of 1 to 6 carbon atoms, and the term "hydrocarbylene" intends a divalent hydrocarbyl moiety containing 1 to about 30 carbon atoms, preferably 1 to about 20 carbon atoms, most preferably 1 to about 12 carbon atoms, including linear, branched, cyclic, saturated and unsaturated species. The term "lower hydrocarbylene" intends a hydrocarbylene group of 1 to 6 carbon atoms. Unless otherwise indicated, the terms "hydrocarbyl" and "hydrocarbylene" are to be interpreted as including bstituted and/or heteroatom-containing hydrocarbyl and hydrocarbylene moieties, jpectively. 9054] The term "heteroatom-containing" as m a "heteroatom-containing alkyl group" so termed a "heteroalkyl" group) or a "heteroatom-containing aryl group" (also termed a ^teroaryl" group) refers to a molecule, linkage, or substituent in which one or more carbon ims are replaced with an atom other than carbon, e.g., nitrogen, oxygen, sulfur, phosphorus silicon, typically nitrogen, oxygen or sulfur. Similarly, the term "heteroalkyl" refers to an ;yl substituent that is heteroatom-containing, the term "heterocyclic" refers to a cyclic 3stituent that is heteroatom-containing, the terms "heteroaryl" and heteroaromatic" pectively refer to "aryl" and "aromatic" substituents that are heteroatom-containing, and : like. Examples of heteroalkyl groups include alkoxyaryl, alkylsulfanyl-substituted alkyl, alkylated amino alkyl, and the like. Examples of heteroaryl substituents include pyrrolyl, iToiidinyl, pyridinyl, quinolinyl, indolyl, pyrimidinyl, hnidazolyl, 1,2,4-triazolyl, tetrazolyl, ., and examples of heteroatom-containing alicyclic groups are pyrrolidine, morpholino, lerazino, piperidino, etc. It should be noted that a "heterocyclic" group or compound may may not be aromatic, and further that "heterocycles" may be monocyclic, bicyclic, or lycyclic as described above with respect to the term "aryl." 1055] By "substituted" as in "substituted hydrocarbyl," "substituted alkyl," ibstituted aryl," and the like, as alluded to in some of the aforementioned definitions, is ant that in the hydrocarbyl, alkyl, aryl, or other moiety, at least one hydrogen atom bound I carbon (or other) atom is replaced with a non-hydrogen substituent. Examples of such )stituents include, without limitation, functional groups such as halide, hydroxyl, fliydryl, Ci-C^o alkoxy, C5-C20 aryloxy, C2-C20 acyl (including C2-C20 alkylcarbonyl '0-alkyI) and Ce-Cio arylcarbonyl (-CO-aryl)), acyloxy (-0-acyl), C2-C20 alkoxycarbonyl (-3)-0-alkyl), C6-C20 aiy'loxycarbonyl (-(CO)-O-aryl), halocarbonyl (-CO)-X where X is 0), C2-C20 alkyl-carbonato (-O-(CO)-O-alk"'^ " ^- ™lcarbonato (-O-(CO)-O-aryl), carboxy (-COOH), carboxylato (-C00~), carbamoyl (-(C0)-NH2), mono-(Ci-C2o alkyl)- substituted carbamoyl (-(CO)-NH(Ci-C2o alkyl)), di-(Ci-C2o alkyl)-substituted carbainoyl -(CO)-N(Ci-C2o alkyl)2), mono-substituted arylcarbamoyl {-(CO)-NH-aryl), thiocarbamoyl (-(CS)-NH2), carbamide (-NH-(C0)-NH2), cyano(-C=N), cyanato (-O-ON), formyl (-(CO)- H), thiofomiyl (-(CS)-H), amino (-NH2), mono- and di-(Ci-C2oalkyl)-substituted amino, mono- and di-(C5-C2o aryl)-substituted amino, C2-C20 alkylamido (-NH-(CO)-alkyl), C6-C20 arylamido (-NH-(CO)-aryl), imino (-CR=NH where R = hydrogen, CpCioalkyl, C5-C2oaryl, C6-C24 alkaryl, C6-C24 aralkyl, etc.), alkylimino (-CR=N(alkyl), where R = hydrogen, alkyl, aryl, alkaryl, etc.), arylimino (-CR=N(aryl), where R = hydrogen, alkyl, aryl, alkaryl, etc.), nitro {-NO2), nitroso (-NO), sulfo (-SO2-OH), sulfonato {-SO2-OI, CpCao alkylsulfanyl (-S- alkyl; also termed "alkylthio"), aiylsulfanyl {-S-aryl; also termed "arylthio"), C1-C20 alkylsulfmyl {-{SO)-alkyl), CrCjo arylsulfinyl (-{SO)-aryl), C,-C2o alkylsulfonyl (-S02-alkyl), C5-C2oarylsulfonyl (-S02-aryl), and thiocarbonyl (=S); and the hydrocarbyl moieties C1-C20 alkyl (preferably Cj-Cig alkyl, more preferably Ci-Ci2 alkyl, most preferably Ci-Cg alkyl), C2-C20 alkenyl (preferably C2-C1K alkenyl, more preferably Cj-Cj; alkenyl, most preferably C2-C6 alkenyl), C2-C20 alkynyl (preferably C2-Cig alkynyl, more preferably C2-C12 alkynyl, most preferably C2-C6 alkynyl), C5-C2oaryl (preferably C5-C]4 aryl), C6-C24 alkaryl (preferably C^-Gig alkaryl), and C6-C24 aralkyl (preferably C6-Cig aralkyl). [00056] In addition, the aforementioned functional groups may, if a particular group permits, be further substituted with one or more additional functional groups or with one or more hydrocarbyl moieties such as those specifically enumerated above. Analogously, the above-mentioned hydrocarbyl moieties may be further substituted with one or more functional groups or additional hydrocarbyl moieties such as those specifically enumerated. [00057] By "substantially free of a particular type of chemical compound is meant that a composition or product contains less 10 wt.% of that chemical compound, preferably less than 5 wt.%, more preferably less than 1 wt.%, and most preferably less than 0.1 wt.%. For instance, the depolymerization product herein is "substantially free of metal contaminants, including metals per se, metal salts, metallic complexes, metal alloys, and organometallic compounds. [00058] "Optional" or "optionally" means that the subsequently described circumstance may or may not occur, so that the description includes instances where the circumstance occurs and instances where it does not. For example, the phrase "optionally substituted" mc^s tliEt a non-hydrogen substituent may or may not be present on a given atom, and, thus, the description includes structures wherein a non-hydrogen substituent is present and structures wherein a non-hydrogen substituent is not present. [00059] Accordingly, the invention features a method for depolymerizing a polymer having a backbone containing electrophilic linkages. The electrophilic linkages may be, for example, ester Imkages (-(CO)-O-), carbonate linkages {-O-(CO)-O)-, urethane linkages (-O-(CO)-NH), substituted urethane linkages {-0-(CO)-NR-, where R is a nonhydrogen substituent such as alkyl, aryl, alkaryl, or the like), amido linkages (-(CO)-NH-), substituted amide linkages (-(CO)-NR- where R is as defined previously, thioester linkages (-(CO)-S-), sulfonic ester linkages (-5(0)2-0-), and the like. Other electrophilic linkages that can be cleaved using nucleophilic reagents will be known to those of ordinary skill in the art of organic chemistry and polymer science and/or can be readily found by reference to the pertinent texts and literature. The polymer undergoing depolymerization may be linear or branched, and may be a homopolymer or copolymer, the latter including random, block, multiblock, and alternating copolymers, terpolymers, and the like. Examples of polymers that can be depolymerized using the methodology of the invention include, without limitation: [00060] poly(alky]ene terephthalates) such as fiber-grade PET (a homopolymer made &om monoethylene glycol and terephthalic acid), bottle-grade PET (a copolymer made based on monoethylene glycol, terephthalic acid, and other comonomers such as isophthalic acid, cyclohexene dimethanol, etc.), poly (butylene terephthalate) (PBT), and poly(hexamethylene terephthalate); [00061] poly(alkylene adipates) such as poly(ethylene adipate), poIy(l ,4-butylene adipate), and poly(hexamethylene adipate); [00062] poly(alkylene suberates) such as poly(ethylene suberate); [00063] poly(alkylene sebacales) such as poly(ethylene sebacate); [00064] poly(e-caprolactone) and poly(P-propiolactone); [00065] poIy(alkylene isophthalates) such as poly(ethylene isophthalate); [00066] poly(alkylene 2,6-naphthalene-dicarboxylates) such as poly(ethylene 2,6- naphthalene-dicarboxylate); [00067] poly(alkylene sulfonyl-4,4'-dibenzoates) such as poly(elhylene sulfonyI-4,4'- dibenzoate); [00068] poly(p-phenylene alkylene dicarboxylates) such as poIy(p-phenylene ethylene dicarhoxylates); [00069] poly(/7-a«j-l,4-cyclohexanediyl alkylene dicarboxylates) such as po\y(tranS' 1,4-cyclohexaiiediyl ethylene dicarboxylate); [00070] poly(l ,4-cyclohexane-dimethylene alkylene dicarboxylates) such as poly(l,4- cyclohexane-dimethylene ethylene dicarboxylate); [00071] poly([2.2.2]-bicyclooctane-l,4-dimethylene alkylene dicarboxylates) such as po]y([2.2.2]-bicyclooctane-l,4-dimethylene ethylene dicarboxylate); [00072] lactic acid polymers and copolymers such as (i?)-polylactide, (J?,5)-polylactide, poIy(tetramethylglycolide), and poly(lactide-co-glycolide); and [00073] polycarbonates of bisphenol A, 3,3'-dimethylbisphenol A, 3,3',5,5'- tetrachlorobisphenol A, 3,3',5,5'-tetramethylbisphenol A; [00074] polyamides such as poly(p-phenylene terephthalamide) (Kevlar®); [00075] poly(aIkylene carbonates) such as poly(propylene carbonate); [00076] polyurethanes such as those available under the tradenames Baytec® and Bayfil®, from Bayer Corporation; and [00077] polyurethane/polyester copolymers such as that available under the tradename Baydar , from Bayer Corporation. [00078] Depolymerization of the polymer is carried out, as indicated above, in the presence of a nucleophilic reagent and a catalyst. Nucleophilic reagents, as will be appreciated by those of ordinary skill in the art, include monohydric alcohols, diols, polyols, thiols, primary amines, and the like, and may contain a single nucleophilic moiety or two or more nucleophilic moieties, e.g., hydroxyl, sulfhydryl, and/or amino groups. The nucleophilic reagent is selected to correspond to the particular electrophilic linkages in the polymer backbone, such that nucleophilic attack at the elecfrophilic linkage results in cleavage of the linkage, For example, a polyester can be cleaved at the ester linkages within the polymer backbone using an alcohol, preferably a primary alcohol, most preferably a C2- C4 monohydric alcohol such as ethanol, isopropanol, and t-butyl alcohol. It will be appreciated that such a reaction cleaves the ester linkages via a transesterification reaction, as will be illustrated infra. [00079] The preferred catalysts for the depolymerization reaction are carbenes and ^^^ carbene precursors. Carbenes include, for instance, diarylcarbenes, cyclic diaminocarbenes, imidazol-2-ylidenes, 1,2,4-triazol-3-yHdenes, l,3-thiazol-2-ylidenes, acyclic diaminQcarbenes, acyclic aminooxycarbenes, acyclic aminothiocarbenes, cyclic diborylcarbenes, acyclic diborylcarbenes, phosphinosilylcarbenes, phosphinophosphoniocarbenes, sulfenyl-trifluoromethylcarbene, and sulfenyl-pentafiuorothiocarbene. See Bourissou et al. (2000), cited supra. Preferred carbenes are heteroatom-stabilized carbenes and preferred carbene precursors are precmsors lo heteroatom-stabilized cartienes. nitrogen-containing carbenes, with N-heterocyclic carbenes most preferred. [00080] In one embodiment, heteroatom-stabilized carbenes suitable as depolymerization catalysts herein have the structure of formula (I) wherein the various substituents are as follows: 100081] E' and E^ are independently selected from N, NR^, O, P, PR^, and S, R^ is hydrogen, heteroalkyl, or heteroaryl, and x and y are independently zero, 1, or 2, and are selected to correspond to the valence state of E' and E^, respectively. When E^ and E are other than O or S, then E' and E^ may be linked through a linking moiety that provides a heterocyclic ring in which E' and E are incorporated as heteroatoms. In the latter case, the heterocyclic ring may be aliphatic or aromatic, and may contain substituents and/or heteroatoms. Generally, such a cyclic group will contain 5 or 6 ring atoms. [00082] For example, in representative compounds of formula (I): (1) E' isOorSandxisl; (2) E' is N, X is 1, and E^ is linked to E^; (3) E' is N, x is 2, and E' and E^ are not linked; (4) E' is NR^, x is 1, and E' and E^ are not linked; or (5) E^ is NR^, X is zero, and E' is linked to EI [00083] R' and R^ are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30 hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic Cj-Cso hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl. Preferably, at least one of R' and R^, and more preferably both R' and R^, are relatively biilky groups. particularly branched alkyl (including substituted and/or heteroatom-containing alkyl), aryi (including substituted aryl, heteroaiyl, and substituted heteroaryl), alkary! (including substituted and/or heteroatom-containing aralkyl), and alicyclic. Using such sterically bulky groups to protect the highly reactive carbene center has been found to kinetically stabihze singlet carbenes, which are preferred reaction catalysts herein. Particular sterically bulky groups that are suitable as R' and R^ are optionally substituted and/or heteroatom-containing C3-C12 alkyl, tertiary C4-C12 alkyl, C5-C12 aryl, Ce-Cig alkaryl, and C5-Ci2 alicyclic, with Cs-Ci: aryl and C6-C12 alkaryl particularly preferred. The latter substituents are exemplified by phenyl optionally substituted with 1 to 3 substituents selected from lower alkyl, lower alkoxy, and halogen, and thus mclude, for example, p-methylphenyl, 2,6-dimethyIphenyl, and 2,4,6-trimethylphenyl (mesityl). [00084] L' and L are linkers containing 1 to 6 spacer atoms, and are independently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom- containing hydrocarbylene; and m and n are independently zero or 1, meaning that each of L' and L^ is optional. Preferred L' and L^ moieties include, by way of example, alkylene, alkenylene, arylene, aralkylene, any of which may be heteroatom-containing and/or substituted, or L' and/or I? may be a heteroatom such as O or S, or a substituted heteroatom such as NH, NR (where R is alkyl, aryl, other hydrocarbyl, etc.), or PR; and [00085} In one preferred embodiment, E and E are independently N or NR and are not linked, such that the carbene is an N-heteroacyclic carbene. In another preferred embodiment, E' and E^ are N, x and y are 1, and E^ and E^ are linked through a linking moiety such that the carbene is an N-heterocyclic carbene. N-heterocycIic carbenes suitable herein include, without limitation, compounds having the structure of formula (II) wherein R', R^, L', I?, m, and n are as defined above for carbenes of formula (I). In carbenes of structural formula (II), L is a hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, or substituted heteroatom-containing hydrocarbylene linker, wherein two or more substituents on adjacent atoms within L may be linked to form an additional cyclic group. L is a hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, or substituted heteroatom-contaiiiing hydrocarbylene linker, wherein two or more substituents on adjacent atoms within L may be linked to form an additional cyclic group. For example, L may be -CR^R'-CRR^- or -CR^=CR^-, wherein R\ R^, R^ and R are independently selected from hydrogen, halogen, C1-C12 alkyl, or wherein any two of R , R , R , and R^ may be linked together to form a substituted or unsubstituted, saturated or unsaturated ring. [00086] Accordingly, when L is -CR^R'-CR^R^- or -CR^=CR^-, the carbene has the structure of formula (III) in which q is an optional double bond, s is zero or 1, and 1 is xero or 1, with the proviso that when q is present, s and t are zero, and when q is absent, s and t are 1. [00087] Certain carbenes are new chemical compounds and are claimed as such herein. These are compounds having the structure of formula (I) wherein a heteroatom is directly bound to E' and/or E^. e.g., with the proviso that a heteroatom is directly bound to E', E^, or to both E' and E^, and wlierein the carbene may be m the form of a salt (such that it is positively charged and associated with a negatively charged counterion). These novel carbenes are those wherein a heteroatom is directly bound to E' and/or E , and include, solely r by way of example, carbenes of formula (I) wherein E and/or E isNR and R is a heteroalkyl orheteroaryl group such as an alkoxy, alkylthio, aryloxy, arylthio, aralkoxy, or aralkylthio moiety. Other such carbenes are those wherein x and/or y is at least 1, and L and/or \} is heteroalkyl, heteroaryl, or the like, wherein the heteroatom within L^ and/or \} is directly bound to E' and/or E^, respectively. [00088] Representative of such novel carbenes are compounds of formula (I) wherein E' is NR^, and R^ is alkoxy, substituted alkoxy, aryloxy, substituted aryloxy, aralkoxy, or substituted aralkoxy. A preferred subset of such carbenes are those wherein E is N, x is zero, y is 1, and E' and E^ are linked through a substituted or unsubstituted lower alkylene or lower alkenylene linkage. A more preferred subset of such carbenes are those wherein R^ is lower alkoxy or monocyclic aryl-substituted lower alkoxy, E' and E^ are linked through a moiety -CR^R^-CR^R^- or -CR^=CR^-, wherein R^ R\ R^ and R^ are independently selected from hydrogen, halogen, and Ci-Cu alkyl, n is 1, L^ is lower alkylene, and R^ is monocyclic aryl or substituted monocyclic aryl. Examples 8-11 describe syntheses of representative compounds within this group. [00089] As indicated previously, suitable catalysts for the present depolymerization reaction are also precursors to carbenes, preferably precursors to N-heterocyclic and N-heteroacyclic carbenes. In one embodiment, the precursor is a tri-substituted methane compound having the structure of formula (PI) wherein E', E^, X, y, R', R'^, L', L^, m, and n are as defined for carbenes of structural formula (I), and wherein R' is selected from alkyl, heteroalkyl, aryl, heteroaryl, aralkyl, or heteroaralkyl, and is substituted with at least one electron-withdrawing substituent such as [00090] fluoro, fluoroalkyl (including perfluoroalkyl), chloro, nitre, acytyl. It will be appreciated that the foregoing list is not exhaustive and that any electron-withdrawing group may serve as a substituent providing that the group does not cause unwanted interaction of the catalyst with other components of the depolymerization mixture or adversely affect the depolymerization reaction in any way. Specific examples of R' groups thus mclude p- nitrophenyl, 2,4-dinitrobenzyl, 1,1,2,2-tetrafluoroethyl, pentafluorophenyl, and the like. [00091] Catalysts of formula (PI) are new chemical entities. Representative syntheses of such compounds are described in Examples 13 and 14 herein. As may be deduced from those examples, compounds of formula (PI) wherein E' and E^ are N may be synthesized from the corresponding diamine and an appropriately substituted aldehyde. [00092] Another carbene precursor useful as a catalyst in the present depolymerization reaction has the structure of formula (Pll) [00093] wherein E', E^, X, y, R\ R^ L', V, m, and n are as defined for carbenes of structural formula (I), M is a metal, e.g., gold, silver, other main group metals, or transition metals, with Ag, Cu, Ni, Co, and Fe generally preferred, and Ln is a ligand, generally an anionic or neutral ligand that may or may not be the same as -E'-[(L')m-R']x or -E^-[(L\- R^]y. Generally, carbene precursors of formula (PII) can be synthesized from a carbene salt and a metal oxide; see, e.g., the synthesis described in detail in Example 12. [00094] Still another carbene precursor suitable as a depolymerization catalyst herein is a tetrasubstituted olefm having the structure of formula (PHI) [0009S] wherein: E', E^ X, y, R', R^ L', L^ m, and n are defmed as for carbenes of structural formula (I); E^ and E^ are defmed as for E' and E^; v and w are defmed as for x and y; R^ and R' are defined as for R' and R^; L^ and L' are defmed as for L' and L^; and h and k are defmed as for m and n. These olefins are readily formed from N,N-diaTyl- and N,N-dialkyl-N-heterocyclic carbene salts and a sfrong base, typically an inorganic base such as a metal alkoxide. [00096] As with the carbenesper se, those catalyst precursors having the structure of formula (PII) or (PHI) in which a heteroatom is directly bound to an "E" moiety, i.e., to E', E^, E^, and/or E", are new chemical entities. Preferred such precursors are those wherein the "E" moieties are NR^ or linked N atoms, and the directly boimd heteroatom within R^ is oxygen or suliur. [00097] The depolymerization reaction may be carried out in an inert atmosphere by dissolving a catalytically effective amount of the selected catalyst in a solvent, combining the polymer and the catalyst solution, and then adding the nucleophilic reagent. In a particularly preferred embodiment, however, the polymer, the nucleophilic reagent, and the catalyst (e.g.. a carbene or a carbene precursor) are combined and dissolved in a suitable solvent, and depolymerization thus occurs in a one-step reaction. [00098] Preferably, the reaction mixture is agitated (e.g., stirred), and the progress of the reaction can be monitored by standard techniques, although visual inspection is generally sufficient, insofar as a transparent reaction mixture indicates that the polymer has degraded to an extent sufficient to allow all degradation products to go into solution. Examples of solvents that may be used in the polymerization reaction include organic, protic, or aqueous solvents that are inert under the depolymerization conditions, such as aromatic hydrocarbons, chlorinated hydrocarbons, ethers, aliphatic hydrocarbons, or mixtures thereof Preferred solvents include toluene, methylene chloride, tetrahydrofuran, methyl t-butyl ether, Isopar, gasoline, and mixtures thereof. Supercritical fluids may also be used as solvents, with carbon dioxide representing one such solvent. Reaction temperatures are in the range of about 0 °C to about 100 "^C , typically at most 80 °C, preferably 60 °C or lower, and most preferably 30 °C or less, and the reaction time will generally be in the range of about 12 to 24 hours. Pressures range from atmospheric to pressures typically used in conjunction with supercritical fluids, with the preferred pressure being atmospheric. [00099] It is to be understood that while the invention has been described in conjunction with the preferred specific embodiments thereof, that the foregoing description as well as the examples that follow are mtended to illustrate and not Ihnit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains. EXPERIMENTAL: [000100] General Procedures. 'H and '■'C NMR spectra were recorded on a Bruke- Avance (400 MHz for 'H and 100 MHz for '^C). All NMR spectra were recorded in CDCI3. Materials. Solvents were obtained from Sigma-Aldrich and purified by distillation. Other reagents were obtained commercially or synthesized as follows: po]y(propylene carbonate), poly(bisphenol A carbonate), poly(l,4-butylene adipate), l-ethyl-3-methyI-l-H-imidazoIium chloride, ethylene glycol, butane-2,3-dione monooxime, ammonium hexafluorophosphate, pentafluorobenzaldehyde, and mesityl diamine, obtained from Sigma-Aldrich; l,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene, synthesized according to the method of Arduengo et al. (1999) Tetrahedron 55:14523; N,N-diphenyl imidazoline, chloride salt, synthesized according to the method of Wanzlick et al. {\96\) Angew. Chem. 73:493 and Wanzlick et al. i\9€2)Angew. Chem. 74-128, and Wanzlicketal. (1963) Chem. Ber. 96:3024; 1,3,5-tribenzyl-[l,3,5]triazmane, synthesized according to the method of Arduengo et al. (1992) J. Am. Chem. Soc. 114:5530, cited supra. Example 1 [000101] Depolymerization of Poly(propylene carbonate) (Mw = 50,000) with isolated carbene: 7 mg (0.02 mmol) of l,3-(2,4,6-trimethylphenyl)imida2ol-2-ylidene dissolved in toluene (0.6 mL), was added to a stirred mixture of 0.5 g of poly(propylene carbonate) in toluene (10 mL), under Nj. After stirring for 5 minutes at room temperature, 2 mL of methanol were added to the reaction mixture and the temperature was brought to 80 "C. Stirring was continued for 3 hours followed by the evaporation of the solvent in vacuo. The H and C NMR spectra showed the presence of a single monomer, 4-methyl-[l,3]-dioxolan-2-one. However, there were 4 peaks in the GC-MS. GC-MS: a) m/z ( 5%) 5.099 min = 106 (42), 103 (5), 91 (100), 77 (8), 65 (8), 51 (8) b) m/z (5%) 5.219 min = 106 (60), 105 (30), 103 (8), 91 (100), 77 (8), 65 (5), 51 (5) c) m/z (85%) 6.750 min = 102 (15), 87 (40), 58 (20), 57 (100). Major product. d) m/z (5%) 9.030 min= 136(10), 135 (100), 134 (70), 120 (85), 117 (8), 103 (5), 91 (14), 77 (10), 65 (5). •H NMR :1.4 (d, 3H), 3.9 (t, IH), 4.5 (t, IH), 4.8 (m, IH). '^CNMR: 18.96, 70.42, 73.43,154.88 Example 2 [000102] Depolymerization of Poly(Bisphenol A carbonate) (Mw = 65,000) with isolated carbene: 7 mg (0.02 mmol) of l,3-(2,4,6-trimethylphenyl)imida2ol-2-yHdine dissolved in toluene (1 mL), was added to a stirred mixture of 0.5 g of poIy(bisphenol A carbonate) in toluene (10 mL), under Ni. After stirring for 5 minutes at room temperature, 2 mL of methanol were added to the reaction mixture. The temperature was brought to 80 "C and stirring was continued for 18 hours followed by the evaporation of the solvent in vacuo. The 'H and '^C NMR spectra showed the presence of two compounds identified as, bisphenol A and carbonic acid 4-[l-hydroxy-phenyl)-l-methyl-ethyl]-phenyI ester 4-[l-(4-methoxy-phenyl)-l-methyl-ethyl] phenyl ester. However, GC-MS indicated 4 peaks. GC-MS: a) m/z (5%) 5.]07 min = 106 (40), 103 (5), 91 (100), 77 (8), 65 (8), 51 (8) b) m/z (5%) 5.210 min = 106 (60), 105 (30), 103 (8), 91 (100), 77 (8), 65 (5), 51 (5) c) m/z (60%)14.30] min = 228 (30), 213 (100), 119(15),91 (10). MajorproduCt d) m/z (30%) 16.016 min = 495 (30), 333 (10), 319 (20), 299 (5), 281 (5), 259 (25), 239 (38), 197 (40), 181 (12), 151 (12), 135 (100), 119 (10), 91 (10). 'H NMR : 1.6 - 1.8 (m), 2,4 (s), 3.96 (s), 6.7-6.8 (t), 7.0 - 7.3 (m). Example 3 [000103] Depolymerization of Poly(l,4-butylene adipate) (Mw = 12,000) with isolated carbene: 0.006 g (0.02 mmol) of l,3-(2,4,6-trimethyIphenyl)imidazol-2-ylidine dissolved in toluene (1 mL), was added to a stirred mixture of 1.0 g of poly( 1,4-butylene adipate) in toluene (10 mL), under Nj. After stirring for 5 minutes at room temperature, 2 mL of methanol were added to the reaction mixture. The temperature was brought to 80 °C and stirring was continued for 6 hours followed by the evaporation of the solvent in vacuo. The 'H and '^C NMR showed the presence of a single product, and the GC-MS showed two products. GC-MS: a) m/z(95%) 5.099 min =143 (80), 142(20), 115 (20), 114 (100), lU (70), 101 (65), 87 (12), 83 (25), 82 (12), 74 (36), 73 (26), 69 (10), 59 (72), 55 (60). Major product. b) m/z(5%) 12.199min = 201 (4), 161 (6), 143 (100), 129 (32), 116 (12), 115(25), HI (70), 101 (12), 87 (10), 83 (15), 73 (34), 71 (12), 59 (14), 55 (42). ^H NMR : 1.67 (m), 2.32 (s), 4.08 (s). ^^C NMR : 24.26, 25.18, 33.74, 63.75,173.23 Example 4 [000104] Depolymerization of Poly(propylene carbonate) (Mw = 50,000) with in-situ carbene: To a mixture of 7 mg (0.047 mmol) of l-ethyl-3-methyl-l-H-imidazolium chloride in tetrahydroluran (THF) was added 4 mg (0.038 mmol) of potassium t-butoxide (t-BOK), under N2. After 30 min stirring, 0.1 mL of the reaction mixture was transferred to a flask that was charged with 0.5 g of poIy(propylene carbonate) in 10 mL of THF. The reaction mixture was stirred for 10 min at room temperature followed by the addition of 2 mL of methanol. Stirring was continued at room temperature for 3 hours. Solvent was removed and the 'H and C NMR spectra showed the presence of a single product, 4-methyl-[],3]-dioxolan-2-one. However, before the removal of the solvent the GC-MS of the crude reaction mixture showed 6 different compounds. GC-MS: a) m/z (15%) 6.268 min = 119 (4), 90 (100), 75 (4), 59 (25). b) m/z (5%) 6.451 min = 104 (40), 103 (30), 90 (5), 77 (5), 59 (100), 58 (10), 57 (10). c) m/z (70%) 6.879 min = 102 (10), 87 (25), 58 (14), 57 (100). Major product. d) m/z (1%) 7.565 min = 103 (40), 89 (5), 59 (100), 58 (5), 57 (8). e) m/z (4%) 8.502 min = 207 (14), 133 (10), 103 (35), 90 (10), 89 (10), 59 (100), 58 (12), 57(14). f) m/z (5%) 8.936 min = 148 (8), 118 (8), 117 (15), 103 (20), 77 (60), 72 (8), 59 (100), 58 (5), 57 (5). ^H NMR :1.4 (d, 3H), 3.9 (t, IH), 4.5 (t, IH), 4.8 (m, IH). ^^C NMR : 18.96, 70.42, 73.43,154.88 Example 5 [OOOIOSJ Depolymerization of Poly(bisphenoI A carbonate) (Mv, = 65,000) with in situ carbene: To a mixture of 7 mg (0.047 mmol) of l-ethyl-3-methyl-l-H-imidazolium chloride in THF (1 mL) was added 4 mg (0.038 mmol) of t-BOK, under N2. After 30 min, stirring 0.1 mL of the reaction mixture was transferred to a flask that was charged with 0.5 g of poly(bisphenol A carbonate) in 10 mLof THF. The reaction mixture was stirred for 10 min at room temperature followed by the addition of 2 mL of methanol. Stirring was continued at room temperature for 3 hours. The solvent was removed in vacuo and the H, '^C NMR and GC-MS spectra showed a mixture of monomer and oligomers, where the major product was bisphenol A. GC-MS: a) m/z(10%)12.754min = 212(30),198(20), 197(100), 182(10), 181(10), 179(10), 178 (10), 165 (8), 152 (8), 135 (10), 119 (12), 103 (15), 91 (12), 77 (10), 65 (5). b) m/z (5%) 13.674 min = 282 (5), 281 (10), 255 (8), 229 (10), 228 (40), 214 (20), 213 (100), 208 (3D), 197 (30), 191 (5), 181 (5), 179 (5), 165 (10), 152 (8), 135 (25), 134 (25), 133 (5), 120 (5), 119 (50), 115 (10), 103 (10), 99 (5), 97 (5), 96 (5), 91 (30), 79 (5), 77 (10), 65 (8). c) m/z (35%) 14.286 min = 228 (34), 214 (20), 213 (100), 197 (5), 165 (5), 135 (5), 119 (20), 107 (5), 91 (10), 77 (5), 65 (5). Major Product. d) m/z (35%) 15.189 min = 286 (20), 272 (15), 271 (100), 227 (5), 212 (5), 197 (3), 183 (2), 169 (3), 133 (3), 119 (5). e) m/z (10%) 15.983 min = 344 (20), 330 (20), 329 (100), 285 (5), 269 (3), 226 (3), 211 (2), 183 (3), 165 (3), 153 (2), 133 (6), 121 (2), 91 (2), 77 (1), 59 (3). [000106] Depolymerization of PET according to the above scheme: 20 mg of t-BOK and 45 mg of N,N-diphenyl imidazoline, chloride salt, were placed in a vial with 2 mL THF and stirred for 15 minutes. Ethylene glycol (2.3 g) and PET (0.25 g) (pellets obtained from Aldrich dissolved in CHCI3 and trifluoroacetic acid and precipitated with methanol to form a white powder) were combined to form a PET slurry. The catalyst was added to the slurry with approximately 5 additional mL THF. After 2 hours, the solution became more transparent, indicating dissolution of the components of the depolymerization mixture. The admixture was stirred overnight, yielding a completely clear solution the following day. the THF was removed, yielding 225 mg of white solid. 'H NMR '^C NMR, and GC-MS were all consistent with bis(hydroxy ethylene) terephthalate. [000107] Depolymerization of PET according to the above scheme: 25 mg of 1,3- dimethy] imidazole, iodide salt, and 11 rag of t-BOK were placed in a vial with 2 mL of THF and stirred for 15 min. Methanol (3.11 g) and PET (308 mg, as Jn Example 6) were combined with 5 mL of THF to form an insoluble mixture. The catalyst mixture was filtered into the PET/methanoI mixture. After 1 hour, there was a noticeable increase in transparency. After 14 hours, the solution was completely homogeneous and clear. The solvent was removed by rotary evaporation to yield a white crystalline product (250 mg). 'H NMR indicated complete conversion to dimethyl terephthalate. [000108] Examples 6 and 7 may be better understood by reference to the synthetic route used to prepare the PET and the possible depolymerization products obtained therefrom. The PET obtained in each example was prepared by synthesis according to a two-step transesterification process from dimethyl teraphthalate (DMT) and excess ethylene glycol (EO) in the presence of a metal alkanoate or acetate of calcium, zinc, manganese, titanium etc. The first step generates bis(hydroxy ethylene) teraphthalate (BHET) with the elimination of methanol and the excess EO. The BHET is heated, generally in the presence of a transesterification catalyst, to generate high polymer. This process is generally accomplished in a vented extruder to remove the polycondensate (EO) and generate the desired thermoformed object from a low viscosity precursor. The reaction takes place according to the following scheme: [000109] The different options for chemical recycling are regeneration of the base monomers (DMT and EG), glycolysis of PET back to BHET, decomposition of PET with propylene glycol and reaction of the degradation product with maleic anhydride to form "unsaturated polyesters" for fiber reinforced composites and decomposition with glycols, followed by reaction with dicarboxylic acids to produce polyols for urethane foam and elastomers. [OOOUOj In Example 7, PET powder was slurried in a THF/methanol solvent mixture. N-heterocyclic carbene (3-5 mol%), generated in situ, was added and within approximately 3 hours the PET went into solution. Anaylsis of the degradation product indicated quantitative consumption of PET and depolymerization via transesterification to EO and DMT. The DMT is readily recovered by recrystailization, while EO can be recovered by distillation (Figure 1). Alternatively, and as established in Example 6, if EO is used as the alcohol (-50 to 200mol% excess) in the THF slurry, the depolymerization product is BHET, which is the most desirable and can be directly recycled via conventional methods to PET (Figure 2). The N-heterocycIic carbene catalyst platform is extremely powerful, as the nature of the substituents has a pronounced effect on catalyst stability and activity towards different substrates. (1992), supra.) The resulting mixUire was stirred at room temperature overnight. Removal of the volatiles in vacuo afforded a thick yellow oil of suitable purity in an undetermined yield. ^H-NMR (5, CDCI3): 10.32 (s, IH, N-Ci/-N); 7.39 (m, 5H, Cs/fs); 5.56 (s, 2H, NC//2); 4.38 (s, 3H, OC//3); 2.27 (s, 3H, C//3); 2.20 (s, 3H, CH3). Example 9 [000113] 3-Beiizyl-l-methoxy-4,5-dimethyIimldazolium hexafluorophosphate (3): Crude iodide 2 was taken up in deionized (DI) water, which separated the product &om small amounts of a dark, insoluble residue. The water solution was decanted to a second flask and a solution of ammonium hexafluorophosphate (950 mg, ca. 5.8 mmol) in 10 mL of DI water was added in portions. An oil separated during the addition, and the supernatant solution was decanted out. The oil was crushed in cold (0 °C), and subsequently recrystallized in methanol. Yield: 1.3 g (73 % from 1). 'H-NMR (6, CDCI3): 8.67 (s, IH, N-C//-N); 7.39 (m, 3H, Csi/s); 7.29 (d, 2H, CsHs); 5.24 (s, 2H, NCH2); 4.21 (s, 3H, OCH3); 2.27 (s, 3H, CH^); 2.17 (s, 3H, cm. Example 10 [000114] l-BeDzyloxy-3-benzyl-4,5-dimethyIimidazolium bromide (4): Benzyl bromide (1.2 mL, ca. 10 mmol) was added via syringe to a refluxing suspension of 1 (1.0 g, 5.0 mmol) in dry benzene. A dark orange oil separated after refluxing for 6 h, and cooling to room temperature. The supernatant was decanted and the remaining oil was dried under vacuum overnight, which caused the product to solidify. The solid mass was crushed in pentane, filtered and dried under vacuum. Yield: 1.34 g (63%). 'H-NMR (5, CDCI3): 11.04 (s, IH, N-C//-N); 7.6-7.2 (ov. m, lOH, 2 x CeHi); 5.59, 5.58 (s+s, N-C//2, O-CH2); 2.09, 1.94 (s, 3H, C//3, CHs). 'C-NMR (5, CDCI3): 132.8 (OCHj-'CeHs); 132.5 (NCN); 131.5 (NCHs-'CeHs); 130.6, 130.3,129.2,129.0, 129.0,128.9,128.0 C^^QHs); 124.8; 124.1 (NCCN); 83.9 (OCH2); 51.2 (NCH2); 8.89 (CH3); 7.11 (CH3). [000115] 3-Benzyl-l-beiizyloxy-4,5-dimethyliinidazoliuni hexafluorophosphate (5): A batch of crude bromide 4 (still as an oil before drying under vacuum) was dissolved in DI water and extracted with hexanes. The aqueous layer was separated and a solution of ammonium hexafluorophosphate (ca. 1.3 equiv.) was added dropwise with constant stirring. The yellow oil deposited on the walls of the flask was dissolved in warm methanol and a few drops of hexanes were added. Cooling to room temperature afforded off-white crystals of pure 5, which were rinsed with pentane and dried under vacuum. Yield: (82 % from 1). 'H-NMR (5,CDCl3): 8.42 (s, m,N-C/f-N); 7.45-7.35, 7.18 (ov. m, CeHs); 5.31, 5.20 (s+s,N-CH2, O-CH2); 2.13 (s, 3H, CHi); 2.05 (s, 3H, CH3). [000116] Bis(l-Beiizyloxy-3-benzyl-4,5-dimethyIimidazolylidene)silver(I) dibromoargentate (6). The carbene precursor 6 was prepared as follows: A mixture of silver oxide (128 mg, 0.55 mmol) and imidazolium bromide 4 (396 mg, 1.06 mmol) was taken up in dry CH2CI2 and stirred at room temperature for 90 minutes. The dark orange suspension was filtered through a pad of celite and evaporated to dryness, yielding an orange powder. Crystallization from THF afforded a white powder (2 crops). Yield: 291 mg (57 %). 'H-NMR (5, CD2CI2): 7.47-7.32 (ov. m, lOH, 2 x CeHs); 5.23, 5.22 (s+s, NC//2, OCH2); 2.01, 1.95 (s, 3H+3H, Cft, CH^). '^C-NMR (5, CD2CI2): 136.2 (NCN); 133.3 (OCHz-'CsHs); 130.8 (NCH2-C6Hi); 130.7,130.0; 129.3,129.3,128.5, 127.1, 123.9 C^CeHs + 'NCCN); 82.6 (OCH;); 54.0 (NCH;); 9.4 (CHj); 7.8 (CHg). Anal. Found: C,47.56; H, 4.26; N, 5.79 %. Calc. for C38H4oAg2Br2N402: C, 47.53; H, 4.20;; N, 5.83 %. [000117] Examples 13 and 14 describe preparation of additional carbene precursors from N,N-diaryl-substituted diamines as illustrated in the schemes below. [OOOllS] Synthesis of carbene precursor 7 (2-pentafluorophenyl-l,3-diphenyl- imidazolidine): 200 mg (0.94 mmol, FW = 212.12)N,N'-diphenyl-ethane-l,2-diamine was placed in a vial and dissolved in 5mL CH2CI2. A catalytic amount of p-to!uenesuIfonic acid and 50 mg of Na2S04 were added, followed by 230 mg (0.94 mmol, FW = 196.07) of pentafluorobenzaldehyde. The mixture was stirred for 8h. The Na2S04 was filtered off and solvent was removed under reduced pressure to yield a light brown powder 395 mg (FW = 436.2), 96% yield. 'HNMR: (400 MHZ, CDCI3, 25 °C) *5= 3.7-3.9 (m, 2H), 3.9-4.1 (m, 2H), 6.5 (s, IH), 6.7-6.8 (m, 2H), 6.8-6.9 (m, IH), 7.2-7.5 (m, 2H). '^F NMR: 5 = -143.2 (s br, 2F), -153.7- -153.8 (m, IF), 161.7- -161.8 (m, 2F). Example 14 [000119] Synthesis of carbene precursor 8 (2-peDtafluorophenyI-13-bis-(2,4,6- trimetfayl-phenyl)-imidazolidine): Mesityldiamine (512 mg, 1.7 mmol) was placed into a vial, equipped with a stirbar, with pentafluorobenzaldehyde (340 mg, 1.7 mmol). Glacial acetic acid (5 mL) was added and the reaction was stirred at room temperature for 24h. The acetic acid was removed under reduced pressure and the product was washed several times with cold methanol to afford the product as a white crystalline solid (543 mg, 65%). 'H NMR: (400 MHz, CDCI3, 25 T) 6 : 2.2 (s, 12H), 2.3 (s, 6H), 3.5-3.6 (m, 2H), 3.9-3.4 (m, 2H), 6.4 (s, IH), 6.9 (s, 4H). '^NMR: -136.3- -136.4 (m, IF), -148.6- -148.7 (m, IF), -155.8- -155.9 (m, IF), -163.0- -163.3 (m, 2F). We Claim: 1. A method for depolymerizing a polymer having a backbone containing electrophilic linkages comprising contacting the polymer with a nucleophilic reagent and a catalyst that yields depolymerization products that are substantially free of metal contaminants. 2. The method as claimed in claim 1, wherein the electrophilic linkages are independently selected from ester linkages, carbonate linkages, urethane linkages, substituted urethane linkages, phosphate linkages, amido linkages, substituted amido linkages, thioester linkages, sulfonate ester linkages, and combinations thereof. 3. The method as claimed in claim 2, wherein at least some of the electrophilic linkages are ester linkages, such that the polymer is a polyester. 4. The method as claimed in claim 3, wherein all of the elecfrophilic linkages are ester linkages, such that the polyester is a homopolymer. 5. The method as claimed in claim 3, wherein at least some of the electrophilic linkages are other than ester linkages, such that the polyester is a copolymer. 6. The method as claimed in claim 3, wherein the nucleophilic reagent is a compound containing at least one nucleophilic moiety selected from hydroxyl groups, amino groups, and sulfhydryl groups. 7. The method as claimed in claim 6, wherein the compound contains one nucleophilic moiety. 8. The method as claimed in claim 7, wherein the nucleophilic moiety is a hydroxyl group. 9. The method as claimed in claim 6, wherein the compound contains two nucleophilic moieties. 10. The method as claimed in claim 9, wherein the nucleophilic moieties are hydroxyl groups. 11. The method as claimed in claim 1, wherein the catalyst is selected from carbenes, carbene precursors, and combinations thereof. 12. The method as claimed in claim 11, wherein the catalyst is a carbine or a carbine precursors or a combination thereof. 13. The method as claimed in claim 12, wherein the carbene has the structure of formula (I) wherein: E1 and E2 are independently selected from N, NRE, O, P, PRE, and S, RE is hydrogen, heteroalkyl, or heteroaryl, x and y are independently zero, 1, or 2, and are selected to correspond to the valence state of E1 and E2, respectively, and wherein when E1 and E1 are other than O or S, then E1 and E2may be linked through a linking moiety that provides a heterocyclic ring in which E' and E2 are incorporated as heteroatoms; R1 and R2 are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30 hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl; L1 and L2 are linkers containing 1 to 6 spacer atoms, and are independently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene; and m and n are independently zero or 1. 14. The method as claimed in claim 13, wherein E1 and E2 are N. 15. The method as claimed in claim 14, wherein x and y are 1, and E1 and E2 ire linked through a linking moiety such that the carbene is an N-heterocyclic :arbene. 16. The method as claimed in claim 15, wherein the N-heterocyclic carbene las the structure of formula (II) ;n) A'herein: R1 and R2 are independently selected from branched C3-C30 hydrocarbyl, ;ubstituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30 lydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C3o hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing ;yclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 lydrocarbyl; L is the linking moiety, and is selected from a hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatora-containing hydrocarbylene linker, wherein two or more substituents on adjacent atoms within L may be linked to form an additional cyclic group; one of L1 and L2 is lower alkylene and the other is lower alkylene or O; and m and n are independently zero or 1. 17. The method as claimed in claim 16, wherein: R1 and R2 are independently selected from secondary C3-C12 alkyl, tertiary C4-C12 alkyl, C5-C12 aryl, substituted C5-C12 aryl, C6-C18 alkaryl, substituted C6-C18 alkaryl, C5-C12 alicyclic, and substituted C5-C12 alicyclic; and L is -CR3R4-CR5R6- or -CR3-CR5-, wherein R3 R4, R5 and R6 are ndependently selected from hydrogen, halogen, C1-C12 alkyl, or wherein any two of R3, R4, R5, and R6 may be linked together to form a substituted or unsubstituted, aturated or unsaturated ring, such that the N-heterocyclic carbene has the structure of formula (III) III) ti which q is an optional double bond. 18. The method as claimed In claim 17, wherein: R' and R^ are independently selected from C5-C12 aryl, mono-, di, and tri-ower alkyl-substituted C5-C12 aryl, C6-C12 alkaryl, and mono-, di, and tri-lower ilkyl-substituted C^-Cn alkaryl; m and n are zero; and R3 and R4 are hydrogen. 19. The method as claimed in claim 13, wherein E and E are independently N or NRE and are not linked, such that the carbene is an N-heteroacyclic carbene. 20. The method as claimed in claim 13, wherein E' is NRE^. 21. The method as claimed in claim 20, wherein: RE is alkoxy, substituted alkoxy, aryloxy, substituted aryloxy, aralkoxy, or substituted aralkoxy; E2 is N; X is zero; y is 1; and E1 and E2 are linked through a substituted or unsubstituted lower alkylene or lower alkenylene linkage. 22. The method as claimed in claim 21, wherein: RE is lower alkoxy or monocyclic aryl-substituted lower alkoxy; E1 and E2 are linked through a moiety -CR3R4-CR5R6- or -CR3=CR5-, wherein R3, R4, R5, and R6 are independently selected from hydrogen, halogen, and C1-C12 alkyl; n is 1; L2 is lower alkylene; and R2 is monocyclic aryl or substituted monocyclic aryl. wherein: E1 and E2 are independently selected from N, NRE, O, P, PRE, and S, RE is hydrogen, heteroalkyi, or heteroaryl, x and y are independently zero, 1, or 2, and are selected to correspond to the valence state of E1 and E2, respectively, and wherein when E1 and E2 are other than O or S, then E1 and E2 may be linked through a linking moiety that provides a heterocyclic ring in which E1 and E2 are mcorporated as heteroatoms; R1 and R2 are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30 hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl; L1 and L2 are linkers containing 1 to 6 spacer atoms, and are independently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene; m and n are independently zero or 1; and R7 is selected from alkyl, heteroalkyi, aryl, heteroaryl, aralkyl, or heteroaralkyl, substituted with at least one electron-withdrawing substituent, and further wherein said contacting is carried out in the presence of a base. 24. The method as claimed in claim 12, wherein the carbene precursor has the structure of formula (PIT) wherein: E1 and E2 are independently selected from N, NRE, O, P, PRE, and S, RE is hydrogen, heteroalkyl, or heteroaryl, x and y are independently zero, 1, or 2, and are selected to correspond to the valence state of E1 and E2, respectively, and wherein when E1 and E2 are other than O or S, then E'1and E2 may be linked through a linking moiety that provides a heterocyclic ring in which E'1and E2 are incorporated as heteroatoms; R1 and R2 are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30 hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl; L and L are linkers containing 1 to 6 spacer atoms, and are independently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene; m and n are independently zero or 1; M is a metal; Ln is a neutral or anionic ligand; and j is the number of ligands bound to M, wherein when j is greater than 1, the Ln may be the same or different. 25. The method as claimed in claim 12, wherein the carbene precursor has the structure of formula (PHI) wherein: E1 E2 E4 and E5 are independently selected from N, NRE O, P, PRE, and S, RE is hydrogen, heteroalkyl, or heteroaryl, x, y, v, and w are independently zero, 1, or 2, and are selected to correspond to the valence state of E1 E2, E4, and E5, respectively, and wherein when E1 and E4 are other than O or S, then E1 and E4 may be linked through a linking moiety to form a heterocyclic ring, and when E2 and E5 are other than O or S, then E2 and E5 may be linked through a linking moiety to form a heterocyclic ring; R', R2, R8, and R9 are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30 hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl; L', L2, L4, and L5 are linkers containing 1 to 6 spacer atoms, and are mdependently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene; and h, k, m, and n are independently zero or 1. 26. The method as claimed in claim I comprising contacting the polymer with a nucleophilic reagent and a catalyst at a temperature of at most 80 °C. 27. The method as claimed in claim 27, wherein the temperature is at most 60 °C. 28. The method as claimed in claim 27, wherein the temperature is at most 30 °C. ^,,,'^29. A heteroatom-stabilized carbene having the structure of formula (I) (I) wherein: E' and E2 are independently selected from N, NRe, P, and PRE RE is hydrogen, heteroalkyl, or heteroaryl, x and y are independently zero, 1, or 2, and are selected to correspond to the valence state of E and E2, respectively, and E1 and E2 are linked through a linking moiety that provides a heterocyclic ring in which E and E2 are incorporated as heteroatoms; R and R are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30 hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl; L' and L2 are linkers containing 1 to 6 spacer atoms, and are independently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene; and m and n are independently zero or 1, with the proviso that a heteroatom is directly bound to E1, E2, or to both E andEl 30. The carbene as claimed in claim 29, wherein E' is NRE. 31. The carbene as claimed in claim 30, wherein RE is alkoxy, substituted alkoxy, aryloxy, substituted aryloxy, aralkoxy, substituted aralkoxy, or a lower alkoxy or monocyclic aryl-substituted lower alkoxy. 32. The carbene as claimed in claim 29, wherein: E2 is N; X is zero; y is 1; and 1 0 E and E are linked through a substituted or unsubstituted lower alkylene or lower alkenylene linkage. 33. The carbene as claimed in claim 32, wherein: E1 and E2 are linked through a moiety -CR3R4'-CR5R6- or -CR3=CR5-, wherein R3, R4', R5, and R6 are independently selected from hydrogen, halogen, and C1-C12 alkyl; n is 1; L2 is lower alkylene; and R2 is monocyclic aryl or substituted monocyclic aryl. 34. The carbene as claimed in claim 33, wherein E1 and E2 are linked through a moiety -CR^=CR^-. 35. The carbene as claimed in claim 34, wherein R^ and R^ are hydrogen. 36. The carbene as claimed in claim 34, wherem R^ and R^ are CpCii alkyl. 37. The carbene as claimed in claim 31, wherein: E2 is N; X is zero; y is 1; and E1 and E2 are linked through a substituted or unsubstituted lower alkylene or Jower alkenylene linkage. 38. The carbene as claimed in claim 37, wherein: E1 and E2 are linked through a moiety -CR3R4-CR5R6- or -CR3-CR5-, wherein R3 R4 R5 and R6 are independently selected from hydrogen, halogen, and C1-C12 alkyl; nis 1; L2 is lower alkylene; and R2 is monocyclic aryl or substituted monocyclic aryl. 1 J 39. The carbene as claimed in claim 29, wherein x and y are 1, and E and E are N and linked through a moiety L, such that the carbene has the structure of formula (II) (11) wherein: L is the linking moiety, and is selected from a hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene linker, wherein two or more substituents on adjacent atoms within L may be linked to form an additional cyclic group; one of L' and L2 is lower alkylene and the other is O; and 43 m and n are independently zero or 1. 40. The carbene as claimed in claim 29, wherein L is -CR3R4-CR5R6- or - CR3=CR5-, wherein R3 R4, R5, and R6 are independently selected from hydrogen, halogen, C1-C12 alkyl, or wherein any two of R3, R4, R5, and R6 may be linked together to form a substituted or unsubstituted, saturated or unsaturated ring, such that the N-heterocycHc carbene has the structure of formula (III) (III) in which q is an optional double bond, s is zero or 1, and t is zero or 1, with the proviso that when q is present, s and t are zero, and when q is absent, s and t are 1. 41. The carbene as claimed in claim 40, wherein R and R are independently selected from secondary C3-C12 alkyl, tertiary C4-C12 alkyl, C5-C12 aryl, substituted C5-C12 aryl, C6-C18 alkaryl, substituted C6-C18 alkaryl, C5-C12 alicyclic, and substituted C5-C12 alicyclic. 42. A carbene precursor having the structure of formula (PI) (PI) wherein: E1 and E2 are independently selected from N, NRe, P, and PRE, RE is hydrogen, heteroalkyl, or heteroaryl, x and y are independently zero, 1, or 2, and are 43 selected to correspond to the valence state of E1 and E2, respectively, and E1 and E2 are linked through a linking moiety that provides a heterocyclic ring in which E1 and E2 are incorporated as heteroatoms; R1 and R2 are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C3o hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl; L' and L2 are linkers containing 1 to 6 spacer atoms, and are independently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene; and m and n are independently zero or 1, with the proviso that a heteroatom is directly bound to E', E^, or to both E' and El 43. A carbene precursor having the structure of formula (PII) (PII) wherein: E' hydrogen, heteroalkyl, or heteroaryl, x and y are independently zero, 1, or 2, and are selected to correspond to the valence state of E'1and E2, respectively, and E1 and E2 are linked through a linking moiety that provides a heterocyclic ring in which E1 and E2 are incorporated as heteroatoms; R1 and R2 are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30 44 hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl; L1 and L2 are linkers containing 1 to 6 spacer atoms, and are independently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substhuted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene; m and n are independently 2ero or 1, M is a metal; Ln is a neutral or anionic ligand; and j is the number of Hgands bound to M, wherein when j is greater than 1, the Ln may be the same or different, with the proviso that a heteroatom is directly bound to E', E^, or to both E' and El

Full Text

CATALYTIC DEFOLVMERIZATION OF FOLVMERS CONTAINING ELECTROPHILIC LINKAGES USING NUCLEOPHILIC REAGENTS
ACKNOWLEDGEMENT of GOVERNMENT SUPPORT
[0001] This invention was made iiJpart with Government support under a grant from
the National Science Foundation (CoOTcrative Agreement No. DMR-980677). Accordingly, the Government may have certain pTghts to this invention.
TECHNICAL FIELD
[0002] This invention relates generally to the depolymerization of polymers, and,
more particularly relates to an organocatalytic method for depolymerizing polymers using nucleophilic reagents.. The invention is applicable in numerous fields, including industrial chemistry and chemical waste disposal, plastics recycling, and manufacturing processes requiring a simple and convenient method for the degradation of polymers.
BACKGROUND ART
[0003] Technological advances of all kinds continue to present many complex
ecological issues. Consequently, waste management and pollution prevention are two very significant challenges of the 21" century. The overwhelming quantity of plastic refuse has significantly contributed to the critical shortage of landfill space faced by many communities. For example, poly(ethylene terephthalate) (poly(oxy-l,2-ethanediyl-oxycarbonyl-l,4-diphenylenecarbonyl); "PET"), a widely used engineering thermoplastic for carpeting, clothing, tire cords, soda bottles and other containers, fihn, automotive applications, electronics, displays, etc. will contribute more than 1 billion pounds of waste to land-fills in 2002 alone. The worldwide production of PET has been growing at an annual rate of 10 % per year, and with the increase in use in electronic and automotive applications, this rate is expected to increase significantly to 15% per year. Interestingly, the precursor monomers represent only about 2% of the petrochemical stream. Moreover, the proliferation of the use of organic solvents, halogenated solvents, water, and energy consumption in addressing the need to recycle commodity polymers such as PET and other polyesters has created the need for envu-onmentally responsible and energy efficient recyclmg processes. See Nadkami (1999) International Fiber Journal 14(3).

[0004] Significant effort has been invested in researching recycling strategies for PET,
and these efforts have produced three commercial options; mechanical, chemical and energy recycling. Energy recycling simply bums the plastic for its calorific content. Mechanical recycling, the most widespread approach, involves grinding the polymer to powder, which is then mixed with "virgin" PET. See Mancini et al. (1999) Materials Research 2(l):33-38. Many chemical companies use this process in order to recycle PET at the rate of approximately 50,000 tons/year per plant. In Europe, all new packaging materials as of 2002 must contain 15% recycled material. However, it has been demonstrated that successive recycling steps cause significant polymer degradation, in turn resulting in a loss of desirable mechanical properties. Recycling using chemical degradation involves a process that depolymerizes a polymer to starting material, or at least to relative short oligomeric components. Clearly, this process is most desirable, but is the most difficult to control since elevated temperature and pressure are required along with a catalyst composed of a strong base, or an organometallic complex such as an organic titanate. See Sako et al. (1997) Proc. ofthe4' Int'l Symposium on Supercritical Fluids, ^^. 107-110. The use of such a catalyst results in significant quantities of undesirable byproducts, and materials processed by these methods are thus generally unsuitable for use in medical materials or food packaging, limiting their utility. Moreover, the energy required to effect depolymerization essentially eliminates sustainability arguments.
[OOOS] Accordingly, there is a need in the art for an improved depolymerization .
method. Ideally, such a method would not involve extreme reaction conditions, use of metallic catalysts, or a process that results in significant quantities of potentially problematic by-products.
DISCLOSURE OF THE INVENTION
[0006] The invention is directed to the aforementioned need in the art, and, as such,
provides an efficient catalytic depolymerization reaction that employs mild conditions, wherein production of undesirable byproducts resultmg from polymer degradation is minimized. The reaction can be carried out at temperatures of at most 80 °C, and, because a nonmetaJlic catalyst is preferably employed, the depolymerization products, in a preferred embodiment, are substantially free of metal contaminants. With many of the caibene

catalysts aisciosea nerein, ine uepuiynicnzaLiuii icia»,Lion can be carried out at a temperature
of at most 60 °C or even 30 °C or lower, i.e., at room temperature.
10007] More specifically, in one aspect of the invention, a method is provided for
depolymerizing a polymer having a backbone containing electrophilic linkages, wherein the method involves contacting the polymer with a nucleophilic reagent and a catalyst at a temperature of less than 80 °C. An important application of this method is in the depolymerization of polyesters, including homopolymeric polyesters (in which ali of the electrophilic linkages are ester linkages) and polyester copolymers (in which a fraction of the electrophilic linkages are ester linkages and the remainder of the electrophilic linkages are other than ester linkages).
[0008] In a related aspect of the invention, a method is provided for depolymerizing a
polymer having a backbone containing electrophilic linkages, wherein the method involves contacting the polymer with a nucleophilic reagent and a catalyst that yields depolymerization products that are substantially free of metal contaminants. The polymer may be, for example, a polyester, a polycarbonate, a polyurethane, or a related polymer, in either homopolymeric or copolymeric form, as indicated above. In this embodiment, in order to provide reaction products that are substantially free of contamination by metals and metal-containing compounds, the catalyst used is a purely organic, normietallic catalyst. Preferred catalysis herein are carbene compounds, which act as nucleophilic catalysts, as well as precursors to carbene compounds, as will be discussed injra. As is well understood in the art, carbenes are electronically neutral compounds containing a divalent carbon atom with only six electrons in its valence shell. Carbenes include, by way of example, cyclic diaminocarbenes, imida2ol-2-ylidenes (e.g., l,3-dimesityl-imidazol-2-ylidene and 1,3-dimesityl-4,5-dihydroiniidazoI-2-yIidene), J,2,4-lria2oI-3-y]idenes, and l,3-thiazo!-2-ylidenes; see Bourissou et al. (2000) Chem. Rev. 100:39-91.
[0009] Since the initial description of the synthesis, isolation, and characterization of
stable carbenes by Arduengo (Arduengo et al. (1991) J Am. Chem, Soc. 113:361; Arduengo et al. (1992) J. Am. Chem, Soc. J_U:5530), the exploration of their chemical reactivity has become a major area of research. See, e.g., Arduengo et al. (I999)^cc. Chem. Res. 32:913; Bourissou etal. (2000), jupra; and Brode (1995) yiwgew. Chem. Int. Ed. Eng. 34:1021. Although carbenes have now been extensively investigated, and have in fact been established as usefiil in many synthetically important reactions, there has been no disclosure or

suggestion to use carbenes as catalysts in nucleophilic depolymerization reactions, i.e., reactions in which a polymer containing electrophilic lijikages is depolymerized with a nucleophilic reagent m the presence of a carbene catalyst.
[00010] Suitable catalysts for use herein thus include heteroatom-stabilized carbenes or
precursors to such carbenes. The heteroatom-stabilized carbenes have the structure of formula (I)

wherein:
[00011 ] E' and E^ are independently selected from N, NR^, O, P, PR^, and S, R^ is
hydrogen, heteroalkyi, or heteroary], x and y are independently zero, I, or 2, and are selected
to correspond to the valence state of E and E , respectively, and wherein when E and E^ are
other than O or S, then E' and E may be linked through a linking moiety that provides a
heterocyclic ring in which E' and E are incorporated as heteroatoms;
[00012] R' and R^ are independently selected from branched C3-C30 hydrocarbyl,
substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30
hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30
hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30
hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl;
[00013] L' and L^ are linkers containing 1 to 6 spacer atoms, and are independently
selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene; and
[00014] m and n are independently zero or 1, such that L' and \} are optional.
100015] Certain carbene catalysts of formula (I) are novel chemical compounds and are
claimed as such herein. These novel carbenes are those wherein a heteroatom is directly bound to E' and/or E^, and mclude, solely by way of example, carbenes of formula (I) wherein E' is NR^ and R^ is a heteroalkyi or heteroaryl group such as an alkoxy, alkylthio, aryloxy, arylthio, aralkoxy, or aralkylthio moiety.

{00016] Carbene precursors suitable as catalysts herein include tri-substituted
methanes having the structure of formula (PI), metal adducts having the structure of formula (PII), and tetrasubstituted olefins having the structure (PHI)

wherein, in formulae (PI) and (PII):
[00017] E^ and E^ are independently selected from N, NR^, O, P, PR^, and S, R^ is
hydrogen, heteroalkyl, or heteroaryl, x and y are independently zero, 1, or 2, and are selected
to correspond to the valence state of E' and E^, respectively^ and wherein when E' and E^ are
other than O or S, then E' and E^ may be linked through a linking moiety that provides a
heterocyclic ring in which E and E are incorporated as heteroatoms;
[00018] R' and R^ are independently selected from branched C3-C30 hydrocarbyl,
substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C^-Cso
hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30
hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30
hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl;

[00019] L' and L^ are linkers containing 1 to 6 spacer atoms, and are independently
selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted
hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-
containing hydrocarbylene;
[00020] m aiid n are independently zero or 1;
[00021] R^ is selected from alkyl, heteroalkyl, aryl, heteroaryl, aralkyl, or heteroaralkyi,
substituted with at least one electron-withdrawing substituent;
[00022] M is a metal;
[00023] Ln i5 a ligand; and
[00024] j is the number of Hgands bound to M.
[00025] In compounds of formula (PHI), the substituents are as follows:
[00026] E^ and E" are defined as for E' and E^
[00027] V and w are defined as for x and y;
[00028] R^ and R^ are defined as for R' and R^;
[00029] L^ and L" are defined as for L' and L^ and
[00030] h and k are defined as for m and n.
[00031] The carbene precursors may be in the form of a salt, in which case the
precursor is positively charged and associated with an anionic counterion, such as a halide
ion (I, Br, CI), a hexafluorophosphate anion, or the like.
[00032] Novel carbene precursors herein include compounds of formula (PI), those
compounds of formula (PII) in which a heteroatom is directly bound to E' and/or E^, and
those compounds of formula (PHI) in which a heteroatom is directly bound to at least one of
E', E^ E^, and E^ and may be in the form of a salt as noted above.
[00033] Ideally, the carbene catalyst used in conjunction with the present
depolymerization reaction is an N-heterocycHc carbene having the structure of formula (II)

[00034] R\ R^, L', L^, m, and n are as defined above; and
[00035] L is a hydrocarbylene, substituted hydrocarbylene, heteroatom-containing
hydrocarbylene, or substituted heteroatom-contaming hydrocarbylene linker, wherein two or

more substituents on adjacent atoms within L may be Jinked to form an additional cyclic group.
[00036] As alluded to above, one important application of the present invention is in
the recycling of polyesters, including, by way of illustration and not limitation: PET; poly (butylene terephthalate) (PBT); poly(alkylene adipate)s and their copolymers; and poly(e-caprolactone). The methodology of the invention provides an efficient means to degrade such polymers into their component monomers and/or relatively short oligomeric fragments without need for extreme reaction conditions or metallic catalysts.
BRIEF DESCRIPTION OF THE DRAWINGS
[00037J Figure I illustrates the organocalalytic depolymerization of PET in the
presence of excess methanol using N-heterocyclic carbene catalyst, as evaluated in Example
7.
{00038] Figure 2 illustrates the organocatalytic depolymerization of PET in the
presence of ethylene glycol using N-heterocyclic carbene catalyst, as evaluated in Example 6.
DETAILED DESCRIPTION OF THE INVENTION
{00039| Unless otherwise mdicated, this invention is not limited to specific polymers,
carbene catalysts, nucleophilic reagents, or depolymerization conditions. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.
[00040] As used in the specification and the appended claims, the singular forms "a,"
"an," and "the" mclude plural referents unless the context clearly dictates otherwise. Thus,
for example, reference to "a polymer" encompasses a combination or mixtiu-e of different
polymers as well as a single polymer, reference to "a catalyst" encompasses both a single
catalyst as well as two or more catalysts used in combination, and the like.
[00041] In this specification and in the claims that follow, reference will be made to a
number of terms, which shall be defined to have the following meanings:
[00042] As used herein, the phrase "having the formula" or "having the structure" is
not intended to be limiting and is used in the same way that the term "comprising" is commonly used.

[00043] The term "alkyl" as used herein refers to a linear, branched, or cyclic saturated
hydrocarbon group typically although not necessarily containing 1 to about 20 carbon atoms, preferably 1 to about 12 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, f-butyl, octyl, decyl, and the like, as well as cycloalkyi groups such as cyclopentyl, cyclohexyl and the like. Generally, although again not necessarily, alkyl groups herein contain 1 to about 12 carbon atoms. The term "lower alkyl" intends an alkyl group of 1 to 6 carbon atoms, and the specific term "cycloalkyi" intends a cyclic alkyl group, typically having 4 to 8, preferably 5 to 7, carbon atoms. The term "substituted alkyl" refers to alkyl substituted with one or more substituent groups, and the terms "heteroatom-containing alkyl" and "heteroalkyl" refer to alkyl in which at least one carbon atom is replaced with a heteroatom. If not otherwise indicated, the terms "alkyl" and "lower alkyl" include Imear, branched, cyclic, unsubstituted, substituted, and/or heteroatom-containing alkyl and lower alky], respectively.
[00044] The term "alkylene" as used herein refers to a difunctional linear, branched, or
cyclic alkyl group, where "alkyl" is as defined above.
[00045] The term "alkenyl" as used herein refers to a linear, branched, or cyclic
hydrocarbon group of 2 to about 20 carbon atoms containing at least one double bond, such as ethenyl, n-propenyl, isopropenyl, n-butenyl, isobutenyl, octenyl, decenyl, tetradecenyl, hexadecenyl, eicosenyl, tetracosenyl, and the like. Preferred alkenyl groups herein contain 2 to about 12 carbon atoms. The term "lower alkenyl" intends an alkenyl group of 2 to 6 carbon atoms, and the specific term "cycloalkenyl" intends a cyclic alkenyl group, preferably havmg 5 to 8 carbon atoms. The term "substituted alkenyl" refers to alkenyl substituted with one or more substituent groups, and the terms "heteroatom-containing alkenyl" and "heteroaikenyl" refer to alkenyl in which at least one carbon atom is replaced with a heteroatom. If not otherwise indicated, the terms "alkenyl" and "lower alkenyl" include linear, branched, cycUc, unsubstituted, substituted, and/or heteroatom-containing alkenyl and lower alkenyl, respectively.
[00046] The term "alkenylene" as used herein refers to a difunctional linear, branched,
or cyclic alkenyl group, where "alkenyl" is as defined above.
[00047] The term "alkoxy" as used herein refers to a group -O-alkyl wherein "alkyl" is
as defined above, and the term "alkylthio" as used herein refers to a group -S-alkyl wherein "alkyl is as defined above.

[00048] The term "aryl" as used herein, and unless otherwise specified, refers to an
aromatic substituent containing a single aromatic ring or multiple aromatic rings that are
fused together, directly linked, or indirectly linked (such that the different aromatic rings are
bound to a common group such as a methylene or ethylene moiety). Preferred aryl groups
contain 5 to 20 carbon atoms and either one aromatic ring or 2 to 4 fused or linked aromatic
rings, e.g., phenyl, naphthyl, biphenyl, and the like, with more preferred aryl groups
containing 1 to 3 aromatic rings, and particularly preferred aryl groups containing 1 or 2
aromatic rings and 5 to 14 carbon atoms. "Substituted aryl" refers to an aryl moiety
substituted with one or more substituent groups, and the terms "heteroatom-containing aryl"
and "heteroaryl" refer to aryl in which at least one carbon atom is replaced with a heteroatom.
Unless otherwise indicated, the terms "aromatic," "aryl," and "arylene" include
heteroaromatic, substituted aromatic, and substituted heteroaromatic species.
[00049] The term "aryloxy" refers to a group -0-aryl wherein "aryl" is as defined
above.
[00050] The term "alkaryl" refers to an aryl group with at least one and typically 1 to 6
alkyl, preferably 1 to 3, aUcyl substituents, and the term "aralkyl" refers to an alkyl group with an aryl substituent, wherein "aryl" and "alkyl" are as defmed above. Alkaryl groups include, for example, p-methylphenyl, 2,4-dimethylphenyl, 2,4,6-trimethylphenyl, and the like. The term "aralkyl" refers to an alkyl group substituted with an ary] moiety, wherein "alkyl" and "aryl" are as defined above.
[00051] The term "alkaryloxy" refers to a group -0-R wherein R is alkaryl, the term
"alkarylthio" refers to a group -S-R wherein R is alkaryl, the term aralkoxy refers to a group
-0-R wherein R is aralkyl, the term "aralkylthio" refers to a group -S-R wherein R is aralkyl.
[00052] The terms "halo," "halide," and "halogen" are used in the conventional sense
to refer to a chloro, bronio, fluoro, or iodo substituent. The terms "haloalkyl," "haloalkenyl," and "haloalkynyl" (or "halogenated alkyl," "halogenated alkenyl," and "halogenated alkynyl") refer to an alkyl, alkenyl, or alkynyl group, respectively, in which at least one of the hydrogen atoms in the group has been replaced with a halogen atom.
[00053] "Hydrocarbyl" refers to univalent hydrocarbyl radicals containing 1 to about
30 carbon atoms, preferably 1 to about 20 carbon atoms, more preferably 1 to about 12 carbon atoms, including linear, branched, cyclic, saturated, and unsaturated species, such as alkyl groups, alkenyl groups, aryl groups, alkaryl groups, and the like. The term "lower

hydrocarbyl" intends a hydrocarbyl group of 1 to 6 carbon atoms, and the term "hydrocarbylene" intends a divalent hydrocarbyl moiety containing 1 to about 30 carbon atoms, preferably 1 to about 20 carbon atoms, most preferably 1 to about 12 carbon atoms, including linear, branched, cyclic, saturated and unsaturated species. The term "lower hydrocarbylene" intends a hydrocarbylene group of 1 to 6 carbon atoms. Unless otherwise indicated, the terms "hydrocarbyl" and "hydrocarbylene" are to be interpreted as including bstituted and/or heteroatom-containing hydrocarbyl and hydrocarbylene moieties, jpectively.
9054] The term "heteroatom-containing" as m a "heteroatom-containing alkyl group"
so termed a "heteroalkyl" group) or a "heteroatom-containing aryl group" (also termed a
^teroaryl" group) refers to a molecule, linkage, or substituent in which one or more carbon
ims are replaced with an atom other than carbon, e.g., nitrogen, oxygen, sulfur, phosphorus
silicon, typically nitrogen, oxygen or sulfur. Similarly, the term "heteroalkyl" refers to an
;yl substituent that is heteroatom-containing, the term "heterocyclic" refers to a cyclic
3stituent that is heteroatom-containing, the terms "heteroaryl" and heteroaromatic"
pectively refer to "aryl" and "aromatic" substituents that are heteroatom-containing, and
: like. Examples of heteroalkyl groups include alkoxyaryl, alkylsulfanyl-substituted alkyl,
alkylated amino alkyl, and the like. Examples of heteroaryl substituents include pyrrolyl,
iToiidinyl, pyridinyl, quinolinyl, indolyl, pyrimidinyl, hnidazolyl, 1,2,4-triazolyl, tetrazolyl,
., and examples of heteroatom-containing alicyclic groups are pyrrolidine, morpholino,
lerazino, piperidino, etc. It should be noted that a "heterocyclic" group or compound may
may not be aromatic, and further that "heterocycles" may be monocyclic, bicyclic, or
lycyclic as described above with respect to the term "aryl."
1055] By "substituted" as in "substituted hydrocarbyl," "substituted alkyl,"
ibstituted aryl," and the like, as alluded to in some of the aforementioned definitions, is ant that in the hydrocarbyl, alkyl, aryl, or other moiety, at least one hydrogen atom bound I carbon (or other) atom is replaced with a non-hydrogen substituent. Examples of such )stituents include, without limitation, functional groups such as halide, hydroxyl, fliydryl, Ci-C^o alkoxy, C5-C20 aryloxy, C2-C20 acyl (including C2-C20 alkylcarbonyl '0-alkyI) and Ce-Cio arylcarbonyl (-CO-aryl)), acyloxy (-0-acyl), C2-C20 alkoxycarbonyl (-3)-0-alkyl), C6-C20 aiy'loxycarbonyl (-(CO)-O-aryl), halocarbonyl (-CO)-X where X is 0), C2-C20 alkyl-carbonato (-O-(CO)-O-alk"'^ *" ^- ™lcarbonato (-O-(CO)-O-aryl),

carboxy (-COOH), carboxylato (-C00~), carbamoyl (-(C0)-NH2), mono-(Ci-C2o alkyl)-
substituted carbamoyl (-(CO)-NH(Ci-C2o alkyl)), di-(Ci-C2o alkyl)-substituted carbainoyl
-(CO)-N(Ci-C2o alkyl)2), mono-substituted arylcarbamoyl {-(CO)-NH-aryl), thiocarbamoyl
(-(CS)-NH2), carbamide (-NH-(C0)-NH2), cyano(-C=N), cyanato (-O-ON), formyl (-(CO)-
H), thiofomiyl (-(CS)-H), amino (-NH2), mono- and di-(Ci-C2oalkyl)-substituted amino,
mono- and di-(C5-C2o aryl)-substituted amino, C2-C20 alkylamido (-NH-(CO)-alkyl), C6-C20
arylamido (-NH-(CO)-aryl), imino (-CR=NH where R = hydrogen, CpCioalkyl, C5-C2oaryl,
C6-C24 alkaryl, C6-C24 aralkyl, etc.), alkylimino (-CR=N(alkyl), where R = hydrogen, alkyl,
aryl, alkaryl, etc.), arylimino (-CR=N(aryl), where R = hydrogen, alkyl, aryl, alkaryl, etc.),
nitro {-NO2), nitroso (-NO), sulfo (-SO2-OH), sulfonato {-SO2-OI, CpCao alkylsulfanyl (-S-
alkyl; also termed "alkylthio"), aiylsulfanyl {-S-aryl; also termed "arylthio"), C1-C20
alkylsulfmyl {-{SO)-alkyl), CrCjo arylsulfinyl (-{SO)-aryl), C,-C2o alkylsulfonyl (-S02-alkyl),
C5-C2oarylsulfonyl (-S02-aryl), and thiocarbonyl (=S); and the hydrocarbyl moieties C1-C20
alkyl (preferably Cj-Cig alkyl, more preferably Ci-Ci2 alkyl, most preferably Ci-Cg alkyl),
C2-C20 alkenyl (preferably C2-C1K alkenyl, more preferably Cj-Cj; alkenyl, most preferably
C2-C6 alkenyl), C2-C20 alkynyl (preferably C2-Cig alkynyl, more preferably C2-C12 alkynyl,
most preferably C2-C6 alkynyl), C5-C2oaryl (preferably C5-C]4 aryl), C6-C24 alkaryl
(preferably C^-Gig alkaryl), and C6-C24 aralkyl (preferably C6-Cig aralkyl).
[00056] In addition, the aforementioned functional groups may, if a particular group
permits, be further substituted with one or more additional functional groups or with one or
more hydrocarbyl moieties such as those specifically enumerated above. Analogously, the
above-mentioned hydrocarbyl moieties may be further substituted with one or more
functional groups or additional hydrocarbyl moieties such as those specifically enumerated.
[00057] By "substantially free of a particular type of chemical compound is meant
that a composition or product contains less 10 wt.% of that chemical compound, preferably less than 5 wt.%, more preferably less than 1 wt.%, and most preferably less than 0.1 wt.%. For instance, the depolymerization product herein is "substantially free of metal contaminants, including metals per se, metal salts, metallic complexes, metal alloys, and organometallic compounds.
[00058] "Optional" or "optionally" means that the subsequently described
circumstance may or may not occur, so that the description includes instances where the circumstance occurs and instances where it does not. For example, the phrase "optionally

substituted" mc^s tliEt a non-hydrogen substituent may or may not be present on a given
atom, and, thus, the description includes structures wherein a non-hydrogen substituent is
present and structures wherein a non-hydrogen substituent is not present.
[00059] Accordingly, the invention features a method for depolymerizing a polymer
having a backbone containing electrophilic linkages. The electrophilic linkages may be, for
example, ester Imkages (-(CO)-O-), carbonate linkages {-O-(CO)-O)-, urethane linkages
(-O-(CO)-NH), substituted urethane linkages {-0-(CO)-NR-, where R is a nonhydrogen
substituent such as alkyl, aryl, alkaryl, or the like), amido linkages (-(CO)-NH-), substituted
amide linkages (-(CO)-NR- where R is as defined previously, thioester linkages (-(CO)-S-),
sulfonic ester linkages (-5(0)2-0-), and the like. Other electrophilic linkages that can be
cleaved using nucleophilic reagents will be known to those of ordinary skill in the art of
organic chemistry and polymer science and/or can be readily found by reference to the
pertinent texts and literature. The polymer undergoing depolymerization may be linear or
branched, and may be a homopolymer or copolymer, the latter including random, block,
multiblock, and alternating copolymers, terpolymers, and the like. Examples of polymers
that can be depolymerized using the methodology of the invention include, without
limitation:
[00060] poly(alky]ene terephthalates) such as fiber-grade PET (a homopolymer made
&om monoethylene glycol and terephthalic acid), bottle-grade PET (a copolymer made based
on monoethylene glycol, terephthalic acid, and other comonomers such as isophthalic acid,
cyclohexene dimethanol, etc.), poly (butylene terephthalate) (PBT), and poly(hexamethylene
terephthalate);
[00061] poly(alkylene adipates) such as poly(ethylene adipate), poIy(l ,4-butylene
adipate), and poly(hexamethylene adipate);
[00062] poly(alkylene suberates) such as poly(ethylene suberate);
[00063] poly(alkylene sebacales) such as poly(ethylene sebacate);
[00064] poly(e-caprolactone) and poly(P-propiolactone);
[00065] poIy(alkylene isophthalates) such as poly(ethylene isophthalate);
[00066] poly(alkylene 2,6-naphthalene-dicarboxylates) such as poly(ethylene 2,6-
naphthalene-dicarboxylate);
[00067] poly(alkylene sulfonyl-4,4'-dibenzoates) such as poly(elhylene sulfonyI-4,4'-
dibenzoate);

[00068] poly(p-phenylene alkylene dicarboxylates) such as poIy(p-phenylene ethylene
dicarhoxylates);
[00069] poly(/7-a«j-l,4-cyclohexanediyl alkylene dicarboxylates) such as po\y(tranS'
1,4-cyclohexaiiediyl ethylene dicarboxylate);
[00070] poly(l ,4-cyclohexane-dimethylene alkylene dicarboxylates) such as poly(l,4-
cyclohexane-dimethylene ethylene dicarboxylate);
[00071] poly([2.2.2]-bicyclooctane-l,4-dimethylene alkylene dicarboxylates) such as
po]y([2.2.2]-bicyclooctane-l,4-dimethylene ethylene dicarboxylate);
[00072] lactic acid polymers and copolymers such as (i?)-polylactide, (J?,5)-polylactide,
poIy(tetramethylglycolide), and poly(lactide-co-glycolide); and
[00073] polycarbonates of bisphenol A, 3,3'-dimethylbisphenol A, 3,3',5,5'-
tetrachlorobisphenol A, 3,3',5,5'-tetramethylbisphenol A;
[00074] polyamides such as poly(p-phenylene terephthalamide) (Kevlar®);
[00075] poly(aIkylene carbonates) such as poly(propylene carbonate);
[00076] polyurethanes such as those available under the tradenames Baytec® and
Bayfil®, from Bayer Corporation; and
[00077] polyurethane/polyester copolymers such as that available under the tradename
Baydar , from Bayer Corporation.
[00078] Depolymerization of the polymer is carried out, as indicated above, in the
presence of a nucleophilic reagent and a catalyst. Nucleophilic reagents, as will be
appreciated by those of ordinary skill in the art, include monohydric alcohols, diols, polyols,
thiols, primary amines, and the like, and may contain a single nucleophilic moiety or two or
more nucleophilic moieties, e.g., hydroxyl, sulfhydryl, and/or amino groups. The
nucleophilic reagent is selected to correspond to the particular electrophilic linkages in the
polymer backbone, such that nucleophilic attack at the elecfrophilic linkage results in
cleavage of the linkage, For example, a polyester can be cleaved at the ester linkages within
the polymer backbone using an alcohol, preferably a primary alcohol, most preferably a C2-
C4 monohydric alcohol such as ethanol, isopropanol, and t-butyl alcohol. It will be
appreciated that such a reaction cleaves the ester linkages via a transesterification reaction, as
will be illustrated infra.
[00079] The preferred catalysts for the depolymerization reaction are carbenes and ^^^
carbene precursors. Carbenes include, for instance, diarylcarbenes, cyclic diaminocarbenes,

imidazol-2-ylidenes, 1,2,4-triazol-3-yHdenes, l,3-thiazol-2-ylidenes, acyclic diaminQcarbenes, acyclic aminooxycarbenes, acyclic aminothiocarbenes, cyclic diborylcarbenes, acyclic diborylcarbenes, phosphinosilylcarbenes, phosphinophosphoniocarbenes, sulfenyl-trifluoromethylcarbene, and sulfenyl-pentafiuorothiocarbene. See Bourissou et al. (2000), cited supra. Preferred carbenes are heteroatom-stabilized carbenes and preferred carbene precursors are precmsors lo heteroatom-stabilized cartienes. nitrogen-containing carbenes, with N-heterocyclic carbenes most preferred.
[00080] In one embodiment, heteroatom-stabilized carbenes suitable as
depolymerization catalysts herein have the structure of formula (I)

wherein the various substituents are as follows:
100081] E' and E^ are independently selected from N, NR^, O, P, PR^, and S, R^ is
hydrogen, heteroalkyl, or heteroaryl, and x and y are independently zero, 1, or 2, and are
selected to correspond to the valence state of E' and E^, respectively. When E^ and E are
other than O or S, then E' and E^ may be linked through a linking moiety that provides a
heterocyclic ring in which E' and E are incorporated as heteroatoms. In the latter case, the
heterocyclic ring may be aliphatic or aromatic, and may contain substituents and/or
heteroatoms. Generally, such a cyclic group will contain 5 or 6 ring atoms.
[00082] For example, in representative compounds of formula (I):
(1) E' isOorSandxisl;
(2) E' is N, X is 1, and E^ is linked to E^;

(3) E' is N, x is 2, and E' and E^ are not linked;
(4) E' is NR^, x is 1, and E' and E^ are not linked; or
(5) E^ is NR^, X is zero, and E' is linked to EI
[00083] R' and R^ are independently selected from branched C3-C30 hydrocarbyl,
substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30 hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic Cj-Cso hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl. Preferably, at least one of R' and R^, and more preferably both R' and R^, are relatively biilky groups.

particularly branched alkyl (including substituted and/or heteroatom-containing alkyl), aryi (including substituted aryl, heteroaiyl, and substituted heteroaryl), alkary! (including substituted and/or heteroatom-containing aralkyl), and alicyclic. Using such sterically bulky groups to protect the highly reactive carbene center has been found to kinetically stabihze singlet carbenes, which are preferred reaction catalysts herein. Particular sterically bulky groups that are suitable as R' and R^ are optionally substituted and/or heteroatom-containing C3-C12 alkyl, tertiary C4-C12 alkyl, C5-C12 aryl, Ce-Cig alkaryl, and C5-Ci2 alicyclic, with Cs-Ci: aryl and C6-C12 alkaryl particularly preferred. The latter substituents are exemplified by phenyl optionally substituted with 1 to 3 substituents selected from lower alkyl, lower alkoxy, and halogen, and thus mclude, for example, p-methylphenyl, 2,6-dimethyIphenyl, and 2,4,6-trimethylphenyl (mesityl).
[00084] L' and L are linkers containing 1 to 6 spacer atoms, and are independently
selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted
hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-
containing hydrocarbylene; and m and n are independently zero or 1, meaning that each of L'
and L^ is optional. Preferred L' and L^ moieties include, by way of example, alkylene,
alkenylene, arylene, aralkylene, any of which may be heteroatom-containing and/or
substituted, or L' and/or I? may be a heteroatom such as O or S, or a substituted heteroatom
such as NH, NR (where R is alkyl, aryl, other hydrocarbyl, etc.), or PR; and
[00085} In one preferred embodiment, E and E are independently N or NR and are
not linked, such that the carbene is an N-heteroacyclic carbene. In another preferred embodiment, E' and E^ are N, x and y are 1, and E^ and E^ are linked through a linking moiety such that the carbene is an N-heterocyclic carbene. N-heterocycIic carbenes suitable herein include, without limitation, compounds having the structure of formula (II)

wherein R', R^, L', I?, m, and n are as defined above for carbenes of formula (I). In carbenes of structural formula (II), L is a hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, or substituted heteroatom-containing hydrocarbylene linker,

wherein two or more substituents on adjacent atoms within L may be linked to form an additional cyclic group. L is a hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, or substituted heteroatom-contaiiiing hydrocarbylene linker, wherein two or more substituents on adjacent atoms within L may be linked to form an additional cyclic group. For example, L may be -CR^R'-CR*R^- or -CR^=CR^-, wherein R\ R^, R^ and R are independently selected from hydrogen, halogen, C1-C12 alkyl, or wherein any two of R , R , R , and R^ may be linked together to form a substituted or unsubstituted, saturated or unsaturated ring.
[00086] Accordingly, when L is -CR^R'-CR^R^- or -CR^=CR^-, the carbene has the
structure of formula (III)

in which q is an optional double bond, s is zero or 1, and 1 is xero or 1, with the proviso that when q is present, s and t are zero, and when q is absent, s and t are 1.
[00087] Certain carbenes are new chemical compounds and are claimed as such herein.
These are compounds having the structure of formula (I) wherein a heteroatom is directly bound to E' and/or E^. e.g., with the proviso that a heteroatom is directly bound to E', E^, or to both E' and E^, and wlierein the carbene may be m the form of a salt (such that it is positively charged and associated with a negatively charged counterion). These novel carbenes are those wherein a heteroatom is directly bound to E' and/or E , and include, solely r by way of example, carbenes of formula (I) wherein E and/or E isNR and R is a
heteroalkyl orheteroaryl group such as an alkoxy, alkylthio, aryloxy, arylthio, aralkoxy, or aralkylthio moiety. Other such carbenes are those wherein x and/or y is at least 1, and L and/or \} is heteroalkyl, heteroaryl, or the like, wherein the heteroatom within L^ and/or \} is directly bound to E' and/or E^, respectively.
[00088] Representative of such novel carbenes are compounds of formula (I) wherein
E' is NR^, and R^ is alkoxy, substituted alkoxy, aryloxy, substituted aryloxy, aralkoxy, or substituted aralkoxy. A preferred subset of such carbenes are those wherein E is N, x is zero, y is 1, and E' and E^ are linked through a substituted or unsubstituted lower alkylene or lower

alkenylene linkage. A more preferred subset of such carbenes are those wherein R^ is lower alkoxy or monocyclic aryl-substituted lower alkoxy, E' and E^ are linked through a moiety -CR^R^-CR^R^- or -CR^=CR^-, wherein R^ R\ R^ and R^ are independently selected from hydrogen, halogen, and Ci-Cu alkyl, n is 1, L^ is lower alkylene, and R^ is monocyclic aryl or substituted monocyclic aryl. Examples 8-11 describe syntheses of representative compounds within this group.
[00089] As indicated previously, suitable catalysts for the present depolymerization
reaction are also precursors to carbenes, preferably precursors to N-heterocyclic and N-heteroacyclic carbenes. In one embodiment, the precursor is a tri-substituted methane compound having the structure of formula (PI)

wherein E', E^, X, y, R', R'^, L', L^, m, and n are as defined for carbenes of structural formula
(I), and wherein R' is selected from alkyl, heteroalkyl, aryl, heteroaryl, aralkyl, or
heteroaralkyl, and is substituted with at least one electron-withdrawing substituent such as
[00090] fluoro, fluoroalkyl (including perfluoroalkyl), chloro, nitre, acytyl. It will be
appreciated that the foregoing list is not exhaustive and that any electron-withdrawing group
may serve as a substituent providing that the group does not cause unwanted interaction of
the catalyst with other components of the depolymerization mixture or adversely affect the
depolymerization reaction in any way. Specific examples of R' groups thus mclude p-
nitrophenyl, 2,4-dinitrobenzyl, 1,1,2,2-tetrafluoroethyl, pentafluorophenyl, and the like.
[00091] Catalysts of formula (PI) are new chemical entities. Representative syntheses
of such compounds are described in Examples 13 and 14 herein. As may be deduced from
those examples, compounds of formula (PI) wherein E' and E^ are N may be synthesized
from the corresponding diamine and an appropriately substituted aldehyde.
[00092] Another carbene precursor useful as a catalyst in the present depolymerization
reaction has the structure of formula (Pll)

[00093] wherein E', E^, X, y, R\ R^ L', V, m, and n are as defined for carbenes of
structural formula (I), M is a metal, e.g., gold, silver, other main group metals, or transition
metals, with Ag, Cu, Ni, Co, and Fe generally preferred, and Ln is a ligand, generally an
anionic or neutral ligand that may or may not be the same as -E'-[(L')m-R']x or -E^-[(L\-
R^]y. Generally, carbene precursors of formula (PII) can be synthesized from a carbene salt
and a metal oxide; see, e.g., the synthesis described in detail in Example 12.
[00094] Still another carbene precursor suitable as a depolymerization catalyst herein
is a tetrasubstituted olefm having the structure of formula (PHI)

[0009S] wherein: E', E^ X, y, R', R^ L', L^ m, and n are defmed as for carbenes of
structural formula (I); E^ and E^ are defmed as for E' and E^; v and w are defmed as for x and y; R^ and R' are defined as for R' and R^; L^ and L' are defmed as for L' and L^; and h and k are defmed as for m and n. These olefins are readily formed from N,N-diaTyl- and N,N-dialkyl-N-heterocyclic carbene salts and a sfrong base, typically an inorganic base such as a metal alkoxide.
[00096] As with the carbenesper se, those catalyst precursors having the structure of
formula (PII) or (PHI) in which a heteroatom is directly bound to an "E" moiety, i.e., to E', E^, E^, and/or E"*, are new chemical entities. Preferred such precursors are those wherein the "E" moieties are NR^ or linked N atoms, and the directly boimd heteroatom within R^ is oxygen or suliur.
[00097] The depolymerization reaction may be carried out in an inert atmosphere by
dissolving a catalytically effective amount of the selected catalyst in a solvent, combining the polymer and the catalyst solution, and then adding the nucleophilic reagent. In a particularly preferred embodiment, however, the polymer, the nucleophilic reagent, and the catalyst (e.g..

a carbene or a carbene precursor) are combined and dissolved in a suitable solvent, and depolymerization thus occurs in a one-step reaction.
[00098] Preferably, the reaction mixture is agitated (e.g., stirred), and the progress of
the reaction can be monitored by standard techniques, although visual inspection is generally
sufficient, insofar as a transparent reaction mixture indicates that the polymer has degraded to
an extent sufficient to allow all degradation products to go into solution. Examples of
solvents that may be used in the polymerization reaction include organic, protic, or aqueous
solvents that are inert under the depolymerization conditions, such as aromatic hydrocarbons,
chlorinated hydrocarbons, ethers, aliphatic hydrocarbons, or mixtures thereof Preferred
solvents include toluene, methylene chloride, tetrahydrofuran, methyl t-butyl ether, Isopar,
gasoline, and mixtures thereof. Supercritical fluids may also be used as solvents, with carbon
dioxide representing one such solvent. Reaction temperatures are in the range of about 0 °C
to about 100 "^C , typically at most 80 °C, preferably 60 °C or lower, and most preferably
30 °C or less, and the reaction time will generally be in the range of about 12 to 24 hours.
Pressures range from atmospheric to pressures typically used in conjunction with
supercritical fluids, with the preferred pressure being atmospheric.
[00099] It is to be understood that while the invention has been described in
conjunction with the preferred specific embodiments thereof, that the foregoing description as well as the examples that follow are mtended to illustrate and not Ihnit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains. EXPERIMENTAL:
[000100] General Procedures. 'H and '■'C NMR spectra were recorded on a Bruke-
Avance (400 MHz for 'H and 100 MHz for '^C). All NMR spectra were recorded in CDCI3. Materials. Solvents were obtained from Sigma-Aldrich and purified by distillation. Other reagents were obtained commercially or synthesized as follows: po]y(propylene carbonate), poly(bisphenol A carbonate), poly(l,4-butylene adipate), l-ethyl-3-methyI-l-H-imidazoIium chloride, ethylene glycol, butane-2,3-dione monooxime, ammonium hexafluorophosphate, pentafluorobenzaldehyde, and mesityl diamine, obtained from Sigma-Aldrich; l,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene, synthesized according to the method of Arduengo et al. (1999) Tetrahedron 55:14523; N,N-diphenyl imidazoline, chloride salt, synthesized according to the method of Wanzlick et al. {\96\) Angew. Chem. 73:493 and Wanzlick et al.

i\9€2)Angew. Chem. 74-128, and Wanzlicketal. (1963) Chem. Ber. 96:3024; 1,3,5-tribenzyl-[l,3,5]triazmane, synthesized according to the method of Arduengo et al. (1992) J. Am. Chem. Soc. 114:5530, cited supra.
Example 1
[000101] Depolymerization of Poly(propylene carbonate) (Mw = 50,000) with
isolated carbene: 7 mg (0.02 mmol) of l,3-(2,4,6-trimethylphenyl)imida2ol-2-ylidene dissolved in toluene (0.6 mL), was added to a stirred mixture of 0.5 g of poly(propylene carbonate) in toluene (10 mL), under Nj. After stirring for 5 minutes at room temperature, 2 mL of methanol were added to the reaction mixture and the temperature was brought to 80 "C. Stirring was continued for 3 hours followed by the evaporation of the solvent in vacuo. The H and C NMR spectra showed the presence of a single monomer, 4-methyl-[l,3]-dioxolan-2-one. However, there were 4 peaks in the GC-MS. GC-MS:
a) m/z ( 5%) 5.099 min = 106 (42), 103 (5), 91 (100), 77 (8), 65 (8), 51 (8)
b) m/z (5%) 5.219 min = 106 (60), 105 (30), 103 (8), 91 (100), 77 (8), 65 (5), 51 (5)
c) m/z (85%) 6.750 min = 102 (15), 87 (40), 58 (20), 57 (100). Major product.
d) m/z (5%) 9.030 min= 136(10), 135 (100), 134 (70), 120 (85), 117 (8), 103 (5), 91 (14), 77 (10), 65 (5).
•H NMR :1.4 (d, 3H), 3.9 (t, IH), 4.5 (t, IH), 4.8 (m, IH). '^CNMR: 18.96, 70.42, 73.43,154.88
Example 2
[000102] Depolymerization of Poly(Bisphenol A carbonate) (Mw = 65,000) with
isolated carbene: 7 mg (0.02 mmol) of l,3-(2,4,6-trimethylphenyl)imida2ol-2-yHdine dissolved in toluene (1 mL), was added to a stirred mixture of 0.5 g of poIy(bisphenol A carbonate) in toluene (10 mL), under Ni. After stirring for 5 minutes at room temperature, 2 mL of methanol were added to the reaction mixture. The temperature was brought to 80 "C and stirring was continued for 18 hours followed by the evaporation of the solvent in vacuo. The 'H and '^C NMR spectra showed the presence of two compounds identified as, bisphenol A and carbonic acid 4-[l-hydroxy-phenyl)-l-methyl-ethyl]-phenyI ester 4-[l-(4-methoxy-phenyl)-l-methyl-ethyl] phenyl ester. However, GC-MS indicated 4 peaks.

GC-MS:
a) m/z (5%) 5.]07 min = 106 (40), 103 (5), 91 (100), 77 (8), 65 (8), 51 (8)
b) m/z (5%) 5.210 min = 106 (60), 105 (30), 103 (8), 91 (100), 77 (8), 65 (5), 51 (5)
c) m/z (60%)14.30] min = 228 (30), 213 (100), 119(15),91 (10). MajorproduCt
d) m/z (30%) 16.016 min = 495 (30), 333 (10), 319 (20), 299 (5), 281 (5), 259 (25), 239 (38), 197 (40), 181 (12), 151 (12), 135 (100), 119 (10), 91 (10).
'H NMR : 1.6 - 1.8 (m), 2,4 (s), 3.96 (s), 6.7-6.8 (t), 7.0 - 7.3 (m).
Example 3
[000103] Depolymerization of Poly(l,4-butylene adipate) (Mw = 12,000) with
isolated carbene: 0.006 g (0.02 mmol) of l,3-(2,4,6-trimethyIphenyl)imidazol-2-ylidine dissolved in toluene (1 mL), was added to a stirred mixture of 1.0 g of poly( 1,4-butylene adipate) in toluene (10 mL), under Nj. After stirring for 5 minutes at room temperature, 2 mL of methanol were added to the reaction mixture. The temperature was brought to 80 °C and stirring was continued for 6 hours followed by the evaporation of the solvent in vacuo. The 'H and '^C NMR showed the presence of a single product, and the GC-MS showed two products. GC-MS:
a) m/z(95%) 5.099 min =143 (80), 142(20), 115 (20), 114 (100), lU (70), 101 (65), 87 (12), 83 (25), 82 (12), 74 (36), 73 (26), 69 (10), 59 (72), 55 (60). Major product.
b) m/z(5%) 12.199min = 201 (4), 161 (6), 143 (100), 129 (32), 116 (12), 115(25), HI (70), 101 (12), 87 (10), 83 (15), 73 (34), 71 (12), 59 (14), 55 (42).
^H NMR : 1.67 (m), 2.32 (s), 4.08 (s).
^^C NMR : 24.26, 25.18, 33.74, 63.75,173.23
Example 4
[000104] Depolymerization of Poly(propylene carbonate) (Mw = 50,000) with in-situ
carbene: To a mixture of 7 mg (0.047 mmol) of l-ethyl-3-methyl-l-H-imidazolium chloride in tetrahydroluran (THF) was added 4 mg (0.038 mmol) of potassium t-butoxide (t-BOK),
under N2. After 30 min stirring, 0.1 mL of the reaction mixture was transferred to a flask that was charged with 0.5 g of poIy(propylene carbonate) in 10 mL of THF. The reaction mixture was stirred for 10 min at room temperature followed by the addition of 2 mL of methanol.

Stirring was continued at room temperature for 3 hours. Solvent was removed and the 'H and
C NMR spectra showed the presence of a single product, 4-methyl-[],3]-dioxolan-2-one. However, before the removal of the solvent the GC-MS of the crude reaction mixture showed 6 different compounds. GC-MS:
a) m/z (15%) 6.268 min = 119 (4), 90 (100), 75 (4), 59 (25).
b) m/z (5%) 6.451 min = 104 (40), 103 (30), 90 (5), 77 (5), 59 (100), 58 (10), 57 (10).
c) m/z (70%) 6.879 min = 102 (10), 87 (25), 58 (14), 57 (100). Major product.
d) m/z (1%) 7.565 min = 103 (40), 89 (5), 59 (100), 58 (5), 57 (8).
e) m/z (4%) 8.502 min = 207 (14), 133 (10), 103 (35), 90 (10), 89 (10), 59 (100), 58 (12), 57(14).
f) m/z (5%) 8.936 min = 148 (8), 118 (8), 117 (15), 103 (20), 77 (60), 72 (8), 59 (100), 58 (5), 57 (5).
^H NMR :1.4 (d, 3H), 3.9 (t, IH), 4.5 (t, IH), 4.8 (m, IH). ^^C NMR : 18.96, 70.42, 73.43,154.88
Example 5
[OOOIOSJ Depolymerization of Poly(bisphenoI A carbonate) (Mv, = 65,000) with in
situ carbene: To a mixture of 7 mg (0.047 mmol) of l-ethyl-3-methyl-l-H-imidazolium chloride in THF (1 mL) was added 4 mg (0.038 mmol) of t-BOK, under N2. After 30 min, stirring 0.1 mL of the reaction mixture was transferred to a flask that was charged with 0.5 g of poly(bisphenol A carbonate) in 10 mLof THF. The reaction mixture was stirred for 10 min at room temperature followed by the addition of 2 mL of methanol. Stirring was continued at room temperature for 3 hours. The solvent was removed in vacuo and the H, '^C NMR and GC-MS spectra showed a mixture of monomer and oligomers, where the major product was bisphenol A. GC-MS:
a) m/z(10%)12.754min = 212(30),198(20), 197(100), 182(10), 181(10), 179(10), 178 (10), 165 (8), 152 (8), 135 (10), 119 (12), 103 (15), 91 (12), 77 (10), 65 (5).
b) m/z (5%) 13.674 min = 282 (5), 281 (10), 255 (8), 229 (10), 228 (40), 214 (20), 213 (100), 208 (3D), 197 (30), 191 (5), 181 (5), 179 (5), 165 (10), 152 (8), 135 (25), 134 (25), 133

(5), 120 (5), 119 (50), 115 (10), 103 (10), 99 (5), 97 (5), 96 (5), 91 (30), 79 (5), 77 (10), 65 (8).
c) m/z (35%) 14.286 min = 228 (34), 214 (20), 213 (100), 197 (5), 165 (5), 135 (5), 119
(20), 107 (5), 91 (10), 77 (5), 65 (5). Major Product.
d) m/z (35%) 15.189 min = 286 (20), 272 (15), 271 (100), 227 (5), 212 (5), 197 (3), 183 (2), 169 (3), 133 (3), 119 (5).
e) m/z (10%) 15.983 min = 344 (20), 330 (20), 329 (100), 285 (5), 269 (3), 226 (3), 211 (2), 183 (3), 165 (3), 153 (2), 133 (6), 121 (2), 91 (2), 77 (1), 59 (3).

[000106] Depolymerization of PET according to the above scheme: 20 mg of t-BOK
and 45 mg of N,N-diphenyl imidazoline, chloride salt, were placed in a vial with 2 mL THF
and stirred for 15 minutes. Ethylene glycol (2.3 g) and PET (0.25 g) (pellets obtained from
Aldrich dissolved in CHCI3 and trifluoroacetic acid and precipitated with methanol to form a
white powder) were combined to form a PET slurry. The catalyst was added to the slurry
with approximately 5 additional mL THF. After 2 hours, the solution became more
transparent, indicating dissolution of the components of the depolymerization mixture. The
admixture was stirred overnight, yielding a completely clear solution the following day. the
THF was removed, yielding 225 mg of white solid. 'H NMR '^C NMR, and GC-MS were all
consistent with bis(hydroxy ethylene) terephthalate.

[000107] Depolymerization of PET according to the above scheme: 25 mg of 1,3-
dimethy] imidazole, iodide salt, and 11 rag of t-BOK were placed in a vial with 2 mL of THF and stirred for 15 min. Methanol (3.11 g) and PET (308 mg, as Jn Example 6) were combined with 5 mL of THF to form an insoluble mixture. The catalyst mixture was filtered into the PET/methanoI mixture. After 1 hour, there was a noticeable increase in transparency. After 14 hours, the solution was completely homogeneous and clear. The solvent was removed by rotary evaporation to yield a white crystalline product (250 mg). 'H NMR indicated complete conversion to dimethyl terephthalate.
[000108] Examples 6 and 7 may be better understood by reference to the synthetic route
used to prepare the PET and the possible depolymerization products obtained therefrom. The PET obtained in each example was prepared by synthesis according to a two-step transesterification process from dimethyl teraphthalate (DMT) and excess ethylene glycol (EO) in the presence of a metal alkanoate or acetate of calcium, zinc, manganese, titanium etc. The first step generates bis(hydroxy ethylene) teraphthalate (BHET) with the elimination of methanol and the excess EO. The BHET is heated, generally in the presence of a transesterification catalyst, to generate high polymer. This process is generally accomplished in a vented extruder to remove the polycondensate (EO) and generate the desired thermoformed object from a low viscosity precursor. The reaction takes place according to the following scheme:

[000109] The different options for chemical recycling are regeneration of the base
monomers (DMT and EG), glycolysis of PET back to BHET, decomposition of PET with propylene glycol and reaction of the degradation product with maleic anhydride to form "unsaturated polyesters" for fiber reinforced composites and decomposition with glycols, followed by reaction with dicarboxylic acids to produce polyols for urethane foam and elastomers.
[OOOUOj In Example 7, PET powder was slurried in a THF/methanol solvent mixture.
N-heterocyclic carbene (3-5 mol%), generated in situ, was added and within approximately 3 hours the PET went into solution. Anaylsis of the degradation product indicated quantitative consumption of PET and depolymerization via transesterification to EO and DMT. The DMT is readily recovered by recrystailization, while EO can be recovered by distillation (Figure 1). Alternatively, and as established in Example 6, if EO is used as the alcohol (-50 to 200mol% excess) in the THF slurry, the depolymerization product is BHET, which is the most desirable and can be directly recycled via conventional methods to PET (Figure 2). The N-heterocycIic carbene catalyst platform is extremely powerful, as the nature of the substituents has a pronounced effect on catalyst stability and activity towards different substrates.

(1992), supra.) The resulting mixUire was stirred at room temperature overnight. Removal of the volatiles in vacuo afforded a thick yellow oil of suitable purity in an undetermined yield. ^H-NMR (5, CDCI3): 10.32 (s, IH, N-Ci/-N); 7.39 (m, 5H, Cs/fs); 5.56 (s, 2H, NC//2); 4.38 (s, 3H, OC//3); 2.27 (s, 3H, C//3); 2.20 (s, 3H, CH3).
Example 9
[000113] 3-Beiizyl-l-methoxy-4,5-dimethyIimldazolium hexafluorophosphate (3):
Crude iodide 2 was taken up in deionized (DI) water, which separated the product &om small amounts of a dark, insoluble residue. The water solution was decanted to a second flask and a solution of ammonium hexafluorophosphate (950 mg, ca. 5.8 mmol) in 10 mL of DI water was added in portions. An oil separated during the addition, and the supernatant solution was decanted out. The oil was crushed in cold (0 °C), and subsequently recrystallized in methanol. Yield: 1.3 g (73 % from 1). 'H-NMR (6, CDCI3): 8.67 (s, IH, N-C//-N); 7.39 (m, 3H, Csi/s); 7.29 (d, 2H, CsHs); 5.24 (s, 2H, NCH2); 4.21 (s, 3H, OCH3); 2.27 (s, 3H, CH^); 2.17 (s, 3H,
cm.
Example 10
[000114] l-BeDzyloxy-3-benzyl-4,5-dimethyIimidazolium bromide (4): Benzyl
bromide (1.2 mL, ca. 10 mmol) was added via syringe to a refluxing suspension of 1 (1.0 g, 5.0 mmol) in dry benzene. A dark orange oil separated after refluxing for 6 h, and cooling to room temperature. The supernatant was decanted and the remaining oil was dried under vacuum overnight, which caused the product to solidify. The solid mass was crushed in pentane, filtered and dried under vacuum. Yield: 1.34 g (63%). 'H-NMR (5, CDCI3): 11.04 (s, IH, N-C//-N); 7.6-7.2 (ov. m, lOH, 2 x CeHi); 5.59, 5.58 (s+s, N-C//2, O-CH2); 2.09, 1.94 (s, 3H, C//3, CHs). 'C-NMR (5, CDCI3): 132.8 (OCHj-'CeHs); 132.5 (NCN); 131.5 (NCHs-'CeHs); 130.6, 130.3,129.2,129.0, 129.0,128.9,128.0 C^^QHs); 124.8; 124.1 (NCCN); 83.9 (OCH2); 51.2 (NCH2); 8.89 (CH3); 7.11 (CH3).

[000115] 3-Benzyl-l-beiizyloxy-4,5-dimethyliinidazoliuni hexafluorophosphate (5):
A batch of crude bromide 4 (still as an oil before drying under vacuum) was dissolved in DI water and extracted with hexanes. The aqueous layer was separated and a solution of ammonium hexafluorophosphate (ca. 1.3 equiv.) was added dropwise with constant stirring. The yellow oil deposited on the walls of the flask was dissolved in warm methanol and a few drops of hexanes were added. Cooling to room temperature afforded off-white crystals of pure 5, which were rinsed with pentane and dried under vacuum. Yield: (82 % from 1). 'H-NMR (5,CDCl3): 8.42 (s, m,N-C/f-N); 7.45-7.35, 7.18 (ov. m, CeHs); 5.31, 5.20 (s+s,N-CH2, O-CH2); 2.13 (s, 3H, CHi); 2.05 (s, 3H, CH3).

[000116] Bis(l-Beiizyloxy-3-benzyl-4,5-dimethyIimidazolylidene)silver(I)
dibromoargentate (6). The carbene precursor 6 was prepared as follows: A mixture of silver oxide (128 mg, 0.55 mmol) and imidazolium bromide 4 (396 mg, 1.06 mmol) was taken up in dry CH2CI2 and stirred at room temperature for 90 minutes. The dark orange suspension was filtered through a pad of celite and evaporated to dryness, yielding an orange powder. Crystallization from THF afforded a white powder (2 crops). Yield: 291 mg (57 %). 'H-NMR (5, CD2CI2): 7.47-7.32 (ov. m, lOH, 2 x CeHs); 5.23, 5.22 (s+s, NC//2, OCH2); 2.01, 1.95 (s, 3H+3H, Cft, CH^). '^C-NMR (5, CD2CI2): 136.2 (NCN); 133.3 (OCHz-'CsHs); 130.8 (NCH2-*C6Hi); 130.7,130.0; 129.3,129.3,128.5, 127.1, 123.9 C^CeHs + 'NCCN); 82.6 (OCH;); 54.0 (NCH;); 9.4 (CHj); 7.8 (CHg). Anal. Found: C,47.56; H, 4.26; N, 5.79 %. Calc. for C38H4oAg2Br2N402: C, 47.53; H, 4.20;; N, 5.83 %.
[000117] Examples 13 and 14 describe preparation of additional carbene precursors
from N,N-diaryl-substituted diamines as illustrated in the schemes below.

[OOOllS] Synthesis of carbene precursor 7 (2-pentafluorophenyl-l,3-diphenyl-
imidazolidine): 200 mg (0.94 mmol, FW = 212.12)N,N'-diphenyl-ethane-l,2-diamine was placed in a vial and dissolved in 5mL CH2CI2. A catalytic amount of p-to!uenesuIfonic acid and 50 mg of Na2S04 were added, followed by 230 mg (0.94 mmol, FW = 196.07) of pentafluorobenzaldehyde. The mixture was stirred for 8h. The Na2S04 was filtered off and solvent was removed under reduced pressure to yield a light brown powder 395 mg (FW = 436.2), 96% yield. 'HNMR: (400 MHZ, CDCI3, 25 °C) *5= 3.7-3.9 (m, 2H), 3.9-4.1 (m, 2H), 6.5 (s, IH), 6.7-6.8 (m, 2H), 6.8-6.9 (m, IH), 7.2-7.5 (m, 2H). '^F NMR: 5 = -143.2 (s br, 2F), -153.7- -153.8 (m, IF), 161.7- -161.8 (m, 2F).
Example 14

[000119] Synthesis of carbene precursor 8 (2-peDtafluorophenyI-13-bis-(2,4,6-
trimetfayl-phenyl)-imidazolidine): Mesityldiamine (512 mg, 1.7 mmol) was placed into a vial, equipped with a stirbar, with pentafluorobenzaldehyde (340 mg, 1.7 mmol). Glacial acetic acid (5 mL) was added and the reaction was stirred at room temperature for 24h. The acetic acid was removed under reduced pressure and the product was washed several times with cold methanol to afford the product as a white crystalline solid (543 mg, 65%). 'H NMR: (400 MHz, CDCI3, 25 T) 6 : 2.2 (s, 12H), 2.3 (s, 6H), 3.5-3.6 (m, 2H), 3.9-3.4 (m, 2H), 6.4 (s, IH), 6.9 (s, 4H). '^NMR: -136.3- -136.4 (m, IF), -148.6- -148.7 (m, IF), -155.8- -155.9 (m, IF), -163.0- -163.3 (m, 2F).

We Claim:
1. A method for depolymerizing a polymer having a backbone containing electrophilic linkages comprising contacting the polymer with a nucleophilic reagent and a catalyst that yields depolymerization products that are substantially free of metal contaminants.
2. The method as claimed in claim 1, wherein the electrophilic linkages are independently selected from ester linkages, carbonate linkages, urethane linkages, substituted urethane linkages, phosphate linkages, amido linkages, substituted amido linkages, thioester linkages, sulfonate ester linkages, and combinations thereof.
3. The method as claimed in claim 2, wherein at least some of the
electrophilic linkages are ester linkages, such that the polymer is a polyester.
4. The method as claimed in claim 3, wherein all of the elecfrophilic linkages
are ester linkages, such that the polyester is a homopolymer.
5. The method as claimed in claim 3, wherein at least some of the
electrophilic linkages are other than ester linkages, such that the polyester is a
copolymer.
6. The method as claimed in claim 3, wherein the nucleophilic reagent is a compound containing at least one nucleophilic moiety selected from hydroxyl groups, amino groups, and sulfhydryl groups.
7. The method as claimed in claim 6, wherein the compound contains one nucleophilic moiety.

8. The method as claimed in claim 7, wherein the nucleophilic moiety is a
hydroxyl group.
9. The method as claimed in claim 6, wherein the compound contains two
nucleophilic moieties.
10. The method as claimed in claim 9, wherein the nucleophilic moieties are
hydroxyl groups.
11. The method as claimed in claim 1, wherein the catalyst is selected from
carbenes, carbene precursors, and combinations thereof.
12. The method as claimed in claim 11, wherein the catalyst is a carbine or a
carbine precursors or a combination thereof.
13. The method as claimed in claim 12, wherein the carbene has the structure
of formula (I)

wherein:
E1 and E2 are independently selected from N, NRE, O, P, PRE, and S, RE is hydrogen, heteroalkyl, or heteroaryl, x and y are independently zero, 1, or 2, and are selected to correspond to the valence state of E1 and E2, respectively, and wherein when E1 and E1 are other than O or S, then E1 and E2may be linked through a linking moiety that provides a heterocyclic ring in which E' and E2 are incorporated as heteroatoms;
R1 and R2 are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30

hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl;
L1 and L2 are linkers containing 1 to 6 spacer atoms, and are independently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene; and
m and n are independently zero or 1.
14. The method as claimed in claim 13, wherein E1 and E2 are N.
15. The method as claimed in claim 14, wherein x and y are 1, and E1 and E2 ire linked through a linking moiety such that the carbene is an N-heterocyclic :arbene.
16. The method as claimed in claim 15, wherein the N-heterocyclic carbene
las the structure of formula (II)
;n)
A'herein:
R1 and R2 are independently selected from branched C3-C30 hydrocarbyl, ;ubstituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30 lydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C3o hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing ;yclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 lydrocarbyl;

L is the linking moiety, and is selected from a hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatora-containing hydrocarbylene linker, wherein two or more substituents on adjacent atoms within L may be linked to form an additional cyclic group;
one of L1 and L2 is lower alkylene and the other is lower alkylene or O; and
m and n are independently zero or 1.
17. The method as claimed in claim 16, wherein:
R1 and R2 are independently selected from secondary C3-C12 alkyl, tertiary C4-C12 alkyl, C5-C12 aryl, substituted C5-C12 aryl, C6-C18 alkaryl, substituted C6-C18 alkaryl, C5-C12 alicyclic, and substituted C5-C12 alicyclic; and
L is -CR3R4-CR5R6- or -CR3-CR5-, wherein R3 R4, R5 and R6 are ndependently selected from hydrogen, halogen, C1-C12 alkyl, or wherein any two of R3, R4, R5, and R6 may be linked together to form a substituted or unsubstituted, aturated or unsaturated ring,
such that the N-heterocyclic carbene has the structure of formula (III)
III)
ti which q is an optional double bond.
18. The method as claimed In claim 17, wherein:
R' and R^ are independently selected from C5-C12 aryl, mono-, di, and tri-ower alkyl-substituted C5-C12 aryl, C6-C12 alkaryl, and mono-, di, and tri-lower ilkyl-substituted C^-Cn alkaryl;
m and n are zero; and

R3 and R4 are hydrogen.
19. The method as claimed in claim 13, wherein E and E are independently
N or NRE and are not linked, such that the carbene is an N-heteroacyclic carbene.
20. The method as claimed in claim 13, wherein E' is NRE^.
21. The method as claimed in claim 20, wherein:
RE is alkoxy, substituted alkoxy, aryloxy, substituted aryloxy, aralkoxy, or substituted aralkoxy;
E2 is N;
X is zero;
y is 1; and
E1 and E2 are linked through a substituted or unsubstituted lower alkylene or lower alkenylene linkage.
22. The method as claimed in claim 21, wherein:
RE is lower alkoxy or monocyclic aryl-substituted lower alkoxy;
E1 and E2 are linked through a moiety -CR3R4-CR5R6- or -CR3=CR5-, wherein R3, R4, R5, and R6 are independently selected from hydrogen, halogen, and C1-C12 alkyl;
n is 1;
L2 is lower alkylene; and
R2 is monocyclic aryl or substituted monocyclic aryl.

wherein:
E1 and E2 are independently selected from N, NRE, O, P, PRE, and S, RE is hydrogen, heteroalkyi, or heteroaryl, x and y are independently zero, 1, or 2, and are selected to correspond to the valence state of E1 and E2, respectively, and wherein when E1 and E2 are other than O or S, then E1 and E2 may be linked through a linking moiety that provides a heterocyclic ring in which E1 and E2 are mcorporated as heteroatoms;
R1 and R2 are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30 hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl;
L1 and L2 are linkers containing 1 to 6 spacer atoms, and are independently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene;
m and n are independently zero or 1; and
R7 is selected from alkyl, heteroalkyi, aryl, heteroaryl, aralkyl, or heteroaralkyl, substituted with at least one electron-withdrawing substituent, and
further wherein said contacting is carried out in the presence of a base.
24. The method as claimed in claim 12, wherein the carbene precursor has the structure of formula (PIT)

wherein:
E1 and E2 are independently selected from N, NRE, O, P, PRE, and S, RE is hydrogen, heteroalkyl, or heteroaryl, x and y are independently zero, 1, or 2, and are selected to correspond to the valence state of E1 and E2, respectively, and wherein when E1 and E2 are other than O or S, then E'1and E2 may be linked through a linking moiety that provides a heterocyclic ring in which E'1and E2 are incorporated as heteroatoms;
R1 and R2 are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30 hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl;
L and L are linkers containing 1 to 6 spacer atoms, and are independently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene;
m and n are independently zero or 1;
M is a metal;
Ln is a neutral or anionic ligand; and
j is the number of ligands bound to M, wherein when j is greater than 1, the Ln may be the same or different.

25. The method as claimed in claim 12, wherein the carbene precursor has the
structure of formula (PHI)

wherein:
E1 E2 E4 and E5 are independently selected from N, NRE O, P, PRE, and S, RE is hydrogen, heteroalkyl, or heteroaryl, x, y, v, and w are independently zero, 1, or 2, and are selected to correspond to the valence state of E1 E2, E4, and E5, respectively, and wherein when E1 and E4 are other than O or S, then E1 and E4 may be linked through a linking moiety to form a heterocyclic ring, and when E2 and E5 are other than O or S, then E2 and E5 may be linked through a linking moiety to form a heterocyclic ring;
R', R2, R8, and R9 are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30 hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl;
L', L2, L4, and L5 are linkers containing 1 to 6 spacer atoms, and are mdependently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene; and
h, k, m, and n are independently zero or 1.
26. The method as claimed in claim I comprising contacting the polymer with a
nucleophilic reagent and a catalyst at a temperature of at most 80 °C.

27. The method as claimed in claim 27, wherein the temperature is at most 60 °C.
28. The method as claimed in claim 27, wherein the temperature is at most 30 °C. ^,,,'^29. A heteroatom-stabilized carbene having the structure of formula (I)
(I)
wherein:
E' and E2 are independently selected from N, NRe, P, and PRE RE is hydrogen, heteroalkyl, or heteroaryl, x and y are independently zero, 1, or 2, and are selected to correspond to the valence state of E and E2, respectively, and E1 and E2 are linked through a linking moiety that provides a heterocyclic ring in which E and E2 are incorporated as heteroatoms;
R and R are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30 hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl;
L' and L2 are linkers containing 1 to 6 spacer atoms, and are independently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene; and
m and n are independently zero or 1,
with the proviso that a heteroatom is directly bound to E1, E2, or to both E andEl

30. The carbene as claimed in claim 29, wherein E' is NRE.
31. The carbene as claimed in claim 30, wherein RE is alkoxy, substituted
alkoxy, aryloxy, substituted aryloxy, aralkoxy, substituted aralkoxy, or a lower
alkoxy or monocyclic aryl-substituted lower alkoxy.
32. The carbene as claimed in claim 29, wherein:
E2 is N;
X is zero; y is 1; and
1 0
E and E are linked through a substituted or unsubstituted lower alkylene or lower alkenylene linkage.
33. The carbene as claimed in claim 32, wherein:
E1 and E2 are linked through a moiety -CR3R4'-CR5R6- or -CR3=CR5-, wherein R3, R4', R5, and R6 are independently selected from hydrogen, halogen, and C1-C12 alkyl;
n is 1;
L2 is lower alkylene; and
R2 is monocyclic aryl or substituted monocyclic aryl.
34. The carbene as claimed in claim 33, wherein E1 and E2 are linked through
a moiety
-CR^=CR^-.
35. The carbene as claimed in claim 34, wherein R^ and R^ are hydrogen.
36. The carbene as claimed in claim 34, wherem R^ and R^ are CpCii alkyl.

37. The carbene as claimed in claim 31, wherein:
E2 is N;
X is zero; y is 1; and
E1 and E2 are linked through a substituted or unsubstituted lower alkylene or Jower alkenylene linkage.
38. The carbene as claimed in claim 37, wherein:
E1 and E2 are linked through a moiety -CR3R4-CR5R6- or -CR3-CR5-, wherein R3 R4 R5 and R6 are independently selected from hydrogen, halogen, and C1-C12 alkyl;
nis 1;
L2 is lower alkylene; and
R2 is monocyclic aryl or substituted monocyclic aryl.
1 J
39. The carbene as claimed in claim 29, wherein x and y are 1, and E and E
are N and linked through a moiety L, such that the carbene has the structure of
formula (II)
(11)
wherein:
L is the linking moiety, and is selected from a hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene linker, wherein two or more substituents on adjacent atoms within L may be linked to form an additional cyclic group;
one of L' and L2 is lower alkylene and the other is O; and
43

m and n are independently zero or 1.
40. The carbene as claimed in claim 29, wherein L is -CR3R4-CR5R6- or -
CR3=CR5-, wherein R3 R4, R5, and R6 are independently selected from hydrogen,
halogen, C1-C12 alkyl, or wherein any two of R3, R4, R5, and R6 may be linked
together to form a substituted or unsubstituted, saturated or unsaturated ring, such
that the N-heterocycHc carbene has the structure of formula (III)
(III)
in which q is an optional double bond, s is zero or 1, and t is zero or 1, with the proviso that when q is present, s and t are zero, and when q is absent, s and t are 1.
41. The carbene as claimed in claim 40, wherein R and R are independently selected from secondary C3-C12 alkyl, tertiary C4-C12 alkyl, C5-C12 aryl, substituted C5-C12 aryl, C6-C18 alkaryl, substituted C6-C18 alkaryl, C5-C12 alicyclic, and substituted C5-C12 alicyclic.
42. A carbene precursor having the structure of formula (PI)
(PI)
wherein:
E1 and E2 are independently selected from N, NRe, P, and PRE, RE is
hydrogen, heteroalkyl, or heteroaryl, x and y are independently zero, 1, or 2, and are
43

selected to correspond to the valence state of E1 and E2, respectively, and E1 and E2 are linked through a linking moiety that provides a heterocyclic ring in which E1 and E2 are incorporated as heteroatoms;
R1 and R2 are independently selected from branched C3-C30 hydrocarbyl, substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C3o hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl;
L' and L2 are linkers containing 1 to 6 spacer atoms, and are independently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene; and
m and n are independently zero or 1,
with the proviso that a heteroatom is directly bound to E', E^, or to both E' and El
43. A carbene precursor having the structure of formula (PII)
(PII) wherein:
E'
hydrogen, heteroalkyl, or heteroaryl, x and y are independently zero, 1, or 2, and are selected to correspond to the valence state of E'1and E2, respectively, and E1 and E2 are linked through a linking moiety that provides a heterocyclic ring in which E1 and E2 are incorporated as heteroatoms;
R1 and R2 are independently selected from branched C3-C30 hydrocarbyl,
substituted branched C3-C30 hydrocarbyl, heteroatom-containing branched C4-C30
44

hydrocarbyl, substituted heteroatom-containing branched C4-C30 hydrocarbyl, cyclic C5-C30 hydrocarbyl, substituted cyclic C5-C30 hydrocarbyl, heteroatom-containing cyclic C1-C30 hydrocarbyl, and substituted heteroatom-containing cyclic C1-C30 hydrocarbyl;
L1 and L2 are linkers containing 1 to 6 spacer atoms, and are independently selected from heteroatoms, substituted heteroatoms, hydrocarbylene, substhuted hydrocarbylene, heteroatom-containing hydrocarbylene, and substituted heteroatom-containing hydrocarbylene;
m and n are independently 2ero or 1,
M is a metal;
Ln is a neutral or anionic ligand; and
j is the number of Hgands bound to M, wherein when j is greater than 1, the Ln may be the same or different,
with the proviso that a heteroatom is directly bound to E', E^, or to both E' and El

Documents:

1696-chenp-2005 abstract duplicate.pdf

1696-chenp-2005 abstract.pdf

1696-chenp-2005 assignment.pdf

1696-chenp-2005 claims duplicate.pdf

1696-chenp-2005 claims.pdf

1696-chenp-2005 correspondence -others.pdf

1696-chenp-2005 correspondence -po.pdf

1696-chenp-2005 description (complete) duplicate.pdf

1696-chenp-2005 description (complete).pdf

1696-chenp-2005 drawings duplicate.pdf

1696-chenp-2005 drawings.pdf

1696-chenp-2005 form-1.pdf

1696-chenp-2005 form-18.pdf

1696-chenp-2005 form-26.pdf

1696-chenp-2005 form-3.pdf

1696-chenp-2005 form-5.pdf

1696-chenp-2005 others.pdf

1696-chenp-2005 pct search report.pdf

1696-chenp-2005 pct.pdf

1696-chenp-2005 petition.pdf

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Patent Number

220629

Indian Patent Application Number

1696/CHENP/2005

PG Journal Number

29/2008

Publication Date

18-Jul-2008

Grant Date

29-May-2008

Date of Filing

26-Jul-2005

Name of Patentee

THE BOARD OF TRUSTEES OF THE LELAND STANFORD JUNIOR UNIVERSITY

Applicant Address

Inventors:

#	Inventor's Name	Inventor's Address
1	HEDRICK, James, L
2	WAYMOUTH, Robert, M
3	NYCE, Gregory, W
4	KILICKIRAN, Pinar

PCT International Classification Number

C08J 11/10

PCT International Application Number

PCT/US2003/041283

PCT International Filing date

2003-12-23

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	10/330,853	2002-12-26	U.S.A.